Online peer assessment helps students better understand the subject of the course and evaluation standards but also develops critical thinking skills. However, previous research focused mainly on verifying the benefits of online peer assessment and rarely systematically explored the factors affecting its validity. In some qualitative studies, online assessing peers’ ability has been found to be an essential factor influencing the effectiveness of assessment. Therefore, this study aimed to sort out the factors that affect online assessing peers’ ability under the theory of planned behavior (TPB) while clarifying whether attitudes and perceived behavioral control of assessing peers online would positively influence continuance intention and learners’ online assessing peers’ ability. We retrieved 221 valid samples and employed JASP software to conduct SEM analysis on data. We found that attitudes and perceived behavioral control of assessing peers positively affected continuance intention. Continuance intention mediated the relationship between attitudes, online assessing peers’ ability, and perceived behavioral control and online assessing peers’ ability. It is recommended that participants’ attitudes and continuance intention toward assessment be assessed before the implementation of online peer assessment. When students report that peer assessment is unfair, poorly trained assessors, and unclear assessment criteria, teachers must address these issues promptly to prevent a vicious cycle. Future research could look at other factors interfering with online peer assessment, such as course familiarity and teacher feedback on ratings.