As the representative and maintainer of employees’ rights and interests, union plays an important role in solving the actuality that employees won’t say a word even though their opinions and suggestions can make a difference. However, most of the existing studies only focus on the direct relationship between union practice and employee voice, and few scholars have explored the influence mechanism of union practice on employee voice. Guided by the three-pronged approach of the theory of planned behavior, we integrate the typical variables from previous studies on the influencing factors of employee voice, and establish the framework of the joint effect of psychological safety, voice efficacy and felt obligation for constructive change on employee voice, in order to reveal the " black box” between union practice and employee voice.　　Based on the data of 368 employees in 46 enterprises from Jiangsu, Anhui and Shandong provinces, this paper uses structural equation to test the research model. It arrives at the main findings as follows: union practice has a significant positive influence on employee voice, which means the higher the level of union practice is, the more inclined employees are to voice. In addition, psychological safety, voice efficacy and felt obligation for constructive change mediate the relationship between union practice and employee voice. That is to say, union practice can not only improve employees’ psychological safety by protecting their legitimate rights and interests, enhance employees’ voice efficacy through activities such as reflecting reasonable suggestions to the enterprise, freeing them from worries, but also inspire employees’ felt obligation for constructive change, so as to promote employee voice.　　This empirical research contributes to the literature in three important aspects. Firstly, it verifies the promoting effect of union practice on employee voice, enriches the research on the influencing factors of employee voice, and further confirms the theoretical value of union practice as an independent variable, providing some new ideas for the future research on employee voice behavior and union practice. Secondly, this study explains the specific process of union practice influencing employee voice from the perspective of planned behavior theory, making up for the lack of the influence mechanism between union practice and employee voice. Lastly, it promotes the integration of theories and achievements in the research fields such as labor relations, human resource management and organizational behavior, provides theoretical support for enterprises to strengthen the cooperation with union, and impulses the construction of harmonious labor relations.