Abstract: Urban expansion impact on the well-being of the floating population is a "double-edged sword". This paper is based on RUMIC (2009) and uses Instrumental Variable Method to study the influence on the well-being of the floating population and its transmission mechanism. We found that the relationship of the urban scale and the well-being of the floating population is significantly "U" type curve, city municipal district population scale is about 3.4 million people, city scale for the well-being of the floating population effect appeared inflection point. Among them, Income is the main transmission path for urban scale affecting on the well-being of the floating population, city scale by influencing the individual income and indirect effect of impact on the well-being of the floating population accounts for about 64% of the total effect; At the same time, the city scale by increasing productivity and reducing unemployment which might affect the well-being of the floating population accounted for 6.1% of the total effect and indirect effect of about 9%.This article research conclusion is sound, to promote the development of urbanization in our country and safeguard the welfare of the floating population to provide the beneficial enlightenment.

Key words: urban scale, floating population, happiness