In real life, we are often motivated to plan things to be performed at specific times in the future. Some of these intended actions help other individuals, and thus involve time‐based prospective memory (TBPM) under prosocial motivational conditions. Children's social development is very rapid, and they have relatively stable prosocial motivation during school age. Few studies have paid attention to this issue. This study focuses on three aspects of this issue: (1) the impact of prosocial motivation on the TBPM of school‐age children, (2) whether there are sex differences in this effect, and, for the first time, (3) the processing mechanism by which prosocial motivation affects TBPM in school‐age children in the framework of the motivation cognitive model. A total of 112 elementary school students, aged between 8 and 12, participated in the experiment, using a 2 (group: prosocial motivation, control) × 2 (sex: boy, girl) between‐subjects design. The results showed that prosocial motivation can significantly reduce children's time difference of TBPM. However, we found no sex differences in the effect of prosocial motivation on TBPM in the above two indicators. With regard to the processing mechanism, we found that the prosocial motivation group paid more attention to external time information throughout the experiment. However, their internal attention and the effectiveness of attention did not improve. These results partially support the motivation cognitive model. Overall, this study found that prosocial motivation relies mainly on external attention to improve the TBPM performance of school‐age children.