The changes in activity and isozyme of ascorbate peroxidase (APX) in pomelo (Citrus grandis ‘Guanximiyou’) juicy sac were studied at different development stages. The results showed that APX activity increased gradually with juicy sac development and decreased rapidly at aging and rotting stages. The APX isozyme also changed during juicy sac development stages. A new APX isozyme was identified with mobility of 0.66 on 150 days after anthesis, and its expression enhanced with maturity of juicy sac. On 242 days after anthesis, most of APX isozyme disappeared. The APX activity in normal juicy sac was lower than that in granulated juicy sac. It suggested that APX could scavenge active oxygen free radicals and prevent the oxidative damage during pumelo juicy sac granulation stage.