Abstract: Based on the data of "employer-employee" matching survey in Nanhai District, Foshan City, Guangdong Province in 2016, this paper identifies the impact of on-the-job training on the wage effect of labor union, from the perspective of labor union's educational function and attempts to analyze the mechanism between them. The results show that both trade union and on-the-job training have positive wage effect, and the educational function of trade union measured by the interaction between them can further enlarge the wage effect of trade union. Furthermore, we study the existence and net effect of trade union's educational function, and find that the wage premium of educational function comes from the improvement of enterprise performance through mechanism analysis. In addition, heterogeneity analysis shows that trade unions have a wage premium effect on employees with different skill levels, while their educational function mainly affects the low skilled groups. For other benefits, trade union can promote employees to participate in social security and obtain labor protection articles, and reduce the working hours of employees; on-the-job training has no significant effect on employees' participation in social security, but it can increase the probability ratio of employees to obtain labor protection articles and shorten their working hours; while the educational function of trade union has no such effects. The results show that the educational function is one of the means for trade unions to take into account the interests of both employers and employees, then to achieve mutual benefit. This study will help to investigate the effect of trade union more comprehensively and provide suggestions for trade union construction.