Using a longitudinal data set developed from surveys of 1,285 children (and their families), as well as 683 preschools teachers in 382 preschools in rural western China, we examined the association between preschool quality and the development (cognitive and social-emotional skills) of preschool children. The findings showed that 55 % of the sample children were cognitively delayed, and 49 % of the children were social-emotionally delayed at age 3. The rates of delay decreased to less than 40 % when the children were at preschool age. Although other factors were associated with the decline in development delays, according to the findings, preschool attendance respectively led to an improvement of 0.47 SD (p = 0.001) in child cognitive development (i.e., the fall in delays). More specifically, the global quality and the structural quality of preschools improved child cognitive development by 0.11 SD (p = 0.001) and 0.09 SD (p = 0.004), respectively. The findings of the subgroup analyses suggested that children from disadvantaged backgrounds benefited more from being able to attend quality preschools. Preschool quality also helped to improve developmental outcomes (cognitive abilities and social-emotional skills): for children that were developmentally delayed before age 3, but not for those children that were not delayed when they were 3 years old; for children with lower educated parents, but not for those with more highly educated parents; for children that were from relatively poor families, but not for those that were from relatively rich families; and for children that were attending preschools in rural areas, but not for those who were attending a preschool in an urban area.