In response to concerns surrounding excessive homework and off-campus tutoring for students undergoing compulsory education in China, the government implemented the “double reduction” policy to alleviate these burdens. This study aims to examine the mechanisms that underlie the longitudinal relationship between changes in academic pressure and social anxiety among Chinese adolescents under the background of this policy. A total of 2,202 valid questionnaires were obtained, and the sample consisted of 1,234 students who reported having a smartphone (age: M = 14.60, SD = 1.57). They were recruited to complete the changes in leisure time and academic pressure at Time 1, classmate support at Time 1 and Time 2, problematic smartphone use at Time 1 and Time 2, and social anxiety at Time 1, Time 2, and Time 3. The findings indicate that classmate support and problematic smartphone use at Time 2 significantly mediated the association between changes in academic pressure at Time 1 and social anxiety at Time 3. Changes in leisure time at Time 1 also affected these relationships. Specifically, compared to those with decreased leisure time, the relationship between classmate support at Time 2 and problematic smartphone use at Time 2 was stronger among adolescents with increased leisure time, while the relationship between changes in academic pressure at Time 1 and problematic smartphone use at Time 2 was weaker among adolescents with increased leisure time. These results highlight the importance of promoting effective stress coping strategies and peer interactions for adolescents, and also the importance of promoting active parental participation in leisure time management and guidance towards positive coping styles.