How many couples with at least one child under 3 years would like to have another one or more child(ren) in Eastern China and will an in-cash subsidy be conducive to couple's fertility intentions? In sum, only 15.1% of respondents had further fertility intentions (FFI) before learning about the subsidy, and the planned in-cash subsidy policy increased respondents' overall FFI by 8.5%. Fertility has been declining globally and has reached a new low in China. The reasons why the Chinese three-child policy was under-realized, and how couples will react to a planned monthly ¥1000 (€141.2) subsidy policy, are not fully understood. During January and February 2022, a cross-sectional online survey aiming to understand families' expenses of raising a child under 3 years old, and couples' FFI, was conducted. During the survey period, 272510 respondents scanned the QR code. This study reports the findings pertaining to questions on respondents' sociodemographic characteristics, household factors, FFI, and changes in intention from negative to positive after learning about the planned in-cash subsidy. After exclusion, 144893 eligible responses were included. Respondents' FFI, the effect of a planned ¥1000/month*36 months' in-cash subsidy (€5083.2 in total) on people with a negative FFI before the subsidy, and potential reasons for persistent negative FFI after learning about the subsidy were collected through an anonymous online survey. Stepwise binary logistic regression models were used to select associated factors. The potential fertility rate change and government costs were estimated. A stratified analysis by current child number and sensitivity analysis were also conducted. In sum, 15.7% (22804/144893) of respondents were male, 15.1% of respondents reported a positive FFI, and 10.0% (12288/123051) without an FFI at first changed their intention after learning about the planned in-cash subsidy policy. For those who still said 'no FFI', 46.5%, 20.6%, and 14.7% chose pressure on housing status, expenses on children's education, and lack of time or energy for caring for another child as their first reasons. FFI was strongest in participants receiving the most financial support from their parents, i.e. grandparents (OR = 1.73, 95% CI = 1.63-1.84 for the >¥100000/year group), and weakest in those already having two children (OR = 0.23, 95% CI = 0.22-0.24). For those with no FFI before learning about the subsidy policy, respondents with the highest house loan/rent (>¥120000/year, OR = 1.27, 95% CI = 1.18-1.36) were more likely to change their FFI from 'No' to 'Yes', and those with the highest household income (>¥300000/year, OR = 0.65, 95% CI = 0.60-0.71) were least susceptible to the policy. In our study population, about 1843 more births every year and an additional 0.3 children per woman were projected under a conservative estimation. Annual estimated cost at the provincial scale would be ¥817.7 (€115.5) million, about 1.02‰ of the total General Public Budget Revenue in 2022. The findings were generally robust in the stratified analysis and sensitivity analysis. Selection bias and information errors may exist in the online survey responses. The large sample size and detailed further analysis were used to minimize such biases. Fertility intentions in Eastern China are rather low. Policymakers should focus more on financial and childcare burdens for a better realization of the three-child policy, including housing, education and childcare services. An in-cash subsidy, which has never been used in China previously, shows promising potential for increasing FFI. However, the application of such policy should be in line with local conditions for better cost-effectiveness regarding fertility-boosting and fiscal sustainability for the government in the long run. This work was supported by the National Key Research and Development Plan of China (2019YFC0840702). The authors declare no conflict of interests. N/A.