Abstract Background Adolescence is a formative period of social development. Adolescents have experienced considerable changes in their lives due to the COVID-19 pandemic. We conducted a longitudinal study to examine the effects of the COVID-19 pandemic on adolescents’ prosocial attributes and empathy, as well as their longitudinal bilateral relationships. Methods A total of 2,510 students from five junior schools in Sichuan Province were recruited via random cluster sampling. Data were collected in December 2019 (Wave 1, before the outbreak of the pandemic) and July 2020 (Wave 2, during the pandemic) in Chengdu, Sichuan, China. Prosocial attributes and empathy were measured with the Positive Youth Development Scale (GPYDS) subscale and Chinese Empathy Scale, respectively. Results During the pandemic, both empathy and prosocial attributes decreased significantly from 49.89 (9.12) and 49.89 (8.80) before to 48.29 (8.72) and 49.39 (9.26) (p