Science extracurricular activities are important informal learning forms and have significant effects on science learning. However, the mechanism regarding how informal learning will influence student academic performance is still under-addressed. Through using the secondary data of a national assessment in China, this study is to explore the relationship between the science extracurricular activities, learning interests, self-efficacy, and science achievement. Structural Equation Modeling was used to find the mechanism of how students' self-chose and school-organized extracurricular activities influence science achievement through mediating interests and self-efficacy. Chi-square tests were used to find if there was an opportunity gap in terms of the students' engagement in extracurricular activities. It was discovered that the students' volunteer and school-organized participation in science extracurricular activities had a positive and indirect influence on their science achievement through the mediate variables of the students' learning interests and self-efficacy. However, there were opportunity gaps among different groups of students.