1.Background

More than 25 years of the Reform and Opening-upThe Reform and Opening-up（Gaige Kaifang）is an umbrella term covering a whole range of policy areas to re-orient China from a planned system to a market economy.has witnessed large-scale population mobility in China. Currently the “floating” population reached up to 150 millions，more than 10 percent of total population（1.3 billions）. Among all migrants in China，rural-urban migrant laborers must be the largest group，whose population rose to 120 millions in 2004Quoted from a research report of the Ministry of Labor and Social Security in 2005，also see http：//www.cass.net.cn.. There have been a number of researches，which study rural-urba