To help firstborn children in families expecting a second child navigate the role transition more smoothly, we investigated the emotional and behavioral changes of firstborn children during the transition to siblinghood (TTS) and the factors that contribute to these changes. From March to December 2019, a total of 97 firstborn children (Mage=3.00± 0.97, and female = 51) were included in the study through a questionnaire survey of their mothers, and two follow-up visits were conducted in Chongqing, China. Individual in-depth interviews were conducted with 14 mothers. Both quantitative and qualitative results suggest that emotional and behavioral problems of firstborn children tend to increase during TTS, particularly in issues such as anxiety/depression, somatic complaints, withdrawal, sleep problems, attention problems, and aggressive behavior, as well as internalization problems, externalization problems and total problems in the quantitative study (P<0.05). A poor father-child relationship may increase emotional and behavioral problems in firstborn children (P=0.05). Further qualitative analysis found that younger age and outgoing personality of the firstborn child may improve the emotional and behavioral problems. The firstborn children did have more emotional and behavioral problems during TTS. But these problems can be regulated by family factors and their own characteristics.