Learning experiences can affect students' interest in STEM (science, technology, engineering, and mathematics) careers. Applying the social cognitive career theory, this study tested and compared the effect size and effect mechanism of formal learning experiences (FLE) and informal learning experiences (ILE) on 1133 tenth-grade students' interest in STEM careers (ISC) through a paper questionnaire survey. The results of structural equation model analysis showed that: 1) The total effect of ILE on students' ISC is much greater than that of FLE; 2) ILE, STEM self-efficacy (SSE) and STEM careers perceptions (SCP) can directly affect students' ISC; FLE and ILE can also indirectly affect students' ISC through the mediating role of SSE and SCP. The analyses suggest that in order to improve students' ISC, STEM education (especially informal STEM education) should be strengthened, both formal and informal education should pay attention to the cultivation of students' SSE and SCP.