In line with the expectation of the Second Demographic Transition, premarital cohabitation is becoming increasingly common in China. Based on China Family Panel Studies (CFPS) data from 2010 to 2022, this study adopts the Cox proportional hazards regression model to analyze the influence of premarital cohabitation on the stability of first marriage and its mechanisms. This study also tests several competitive hypotheses, including the Trial Marriage Hypothesis, the Experience Hypothesis, the Selectivity Hypothesis, and the Diffusion Hypothesis. The findings show that the risk of divorce at first marriage is significantly higher for those who have experienced premarital cohabitation, and the negative effect of premarital cohabitation on the stability of first marriage is still robust after removing the self-selection effect. The cultural meaning and popularity of premarital cohabitation varies across different groups. Further analysis reveals the moderating effects of birth cohorts and education levels. Overall, premarital cohabitation still has a negative effect on the stability of first marriage in the Chinese context. However, as cohabitation gradually spreads among recent birth cohorts and highly educated groups, its negative influence is weakened, supporting the main argument of the Diffusion Hypothesis.