In recent years, research on students' scientific competencies has generated much discussion. According to the PISA 2015 Assessment and Analytical Framework, both scientific competencies and attitudes towards science -- interest in science, valuing scientific approaches to enquiry (or epistemological beliefs about science), and environmental awareness -- are integral aspects of scientific literacy. However, few studies have focused on the relationships between them. This article examines how students' scientific competencies are affected by their attitudes. Based on a theoretical analysis, the present study argues that the three attitudinal factors have a direct impact on scientific competencies, interest has an immediate influence on the other two attitudes, and epistemological beliefs are directly affected by environmental awareness. To test this hypothesis, a model was constructed to verify such relationships. PISA 2015 assessment instruments were used for data collection. The participants were 25,658 students from different parts of China, containing Beijing, Shanghai, Jiangsu, Guangdong, Taiwan, Hong Kong, and Macao. Structural Equation Modelling (SEM) was employed for data analysis. The results confirmed our hypothesis. The way in which Chinese students' attitudes affected their scientific competencies did not differ (to a statistically significant level) by gender or district. The limitations and implications of this research are discussed below.