Objective To explore the association between nap duration and cognitive decline using a joint model of longitudinal data in China's aging population. Methods Variables from the China Health and Retirement Longitudinal Study (CHARLS) were selected using recursive feature elimination. A multivariate Cox proportional hazards model was used to assess the effect of baseline napping on cognitive decline. A joint model integrating longitudinal data and time-to-event data was constructed to examine the association between nap duration and cognitive decline. Results This study included 10 314 participants, with a total of 40 579 follow-up records, and the median follow-up time was 10 (8, 10) years. A total of 2 975 (28.8%) people developed cognitive decline. Based on the recursive feature elimination method, 4 key independent variables were selected and included, namely frailty index, age, education level and social participation level. The multivariate Cox proportional hazards model showed that nap duration at baseline had no effect on cognitive function outcomes (P＝0.453). The joint model showed that longitudinal changes in nap duration were significantly associated with cognitive decline (hazard ratio＝1.112, 95% confidence interval 1.099-1.126). Conclusion Based on the joint model, it is found that nap duration is associated with cognitive decline, longer nap duration is associated with an increased risk of cognitive decline, and long nap duration is a risk factor for cognitive decline. The longitudinal data joint model can provide methodological support for the analysis of the association between risk factors and related health problems.