As STEM (Science, Technology, Engineering and Mathematics) education gets more and more attention from society, students' attitudes towards STEM are increasingly concerned. However, there was scant research that has empirically documented the relations among STEM attitude, learning motivation and metacognition. This research used Structural Equation Modeling to examine the above relations. Data were collected from 845 primary school students from grade 4 to 6, regarding their STEM attitude, learning motivation, metacognition, and their sociodemographic characteristics (family socioeconomic status). The results showed that metacognition played a mediating role in the effect of learning motivation on STEM attitude. The family socioeconomic status played a moderating role in the effect of metacognition on STEM attitude, and students with higher family socioeconomic status had a stronger effect on STEM attitude. This research also suggested that it is worth considering the improvement of students' learning motivation to facilitate STEM attitude through promoting their metacognitive skills, meanwhile balancing the gap between students under the difference of socioeconomic level in STEM education.