Project-based learning (PBL) aims to foster students' problem-solving ability and collaboration by engaging them in completing a project in teams. Researchers have indicated the importance of guiding students to make comparisons and reflection during the process of project-based learning. Therefore, the present research proposed a peer assessment-based PBL (PA-PBL) approach to boost students' PBL performances. To assess the effectiveness of the proposed approach, a quasi-experiment was conducted for 3 weeks. A total of 110 secondary students were divided into an experimental group learning with the PA-PBL approach and a control group learning with the conventional PBL (C-PBL) approach. The students' problem-solving tendency, learning achievement, and collaboration tendency were evaluated. The experimental results indicated that the PA-PBL approach improved the students' learning achievement, especially for those students with higher problem-solving tendency. In terms of collaboration tendency, it was found that the PA-PBL approach benefited the lower problem-solving tendency students more than the C-PBL approach, while there was no difference between the higher problem-solving tendency students in the two groups. The experimental results also indicated that the PA-PBL approach could better enhance students' problem-solving tendencies than the C-PBL approach.