Abstract: The income and social security of migrant population is an important practical problem in China. Based on the China Migrants Dynamic Survey from National Health Commission in 2011 and 2017, this study comprehensively evaluates the influence of labor contract on the wage income of the migrant population and explores the similarities and differences of the wage effect of labor contract on labor force of different skill levels by using the propensity score matching method and the heterogeneous treatment effect model that allows multi-layer effects to coexist. The results show that the labor contract has a significant positive impact on the wage of migrant population. With the passage of time, the wage effect of contract is also expanding. Further analysis shows that in labor relations, highly skilled workers are the main group that conclude written labor contracts with employers, and the wage effect of labor contracts also presents an increasing trend with the improvement of labor skills. The labor contract becomes the bargaining chip for employers to screen employees, which damages the rights and interests of low-skilled labor force especially migrant workers, and is not conducive to their citizenization and integration. Therefore, the government should promote the comprehensive implementation of Labor Contract Law, strengthen supervision and punishment，so that all workers can get something from their work，and restore the original intention of labor contract law to protect the legitimate rights and interests of vulnerable groups.

Keywords: Labor Contract; Low-Skilled; Human Capital ; Wage Gap