Abstract: Migrant workers in China are willing to work extra hours.They have double futures of modern workers and traditional peasants,and the logics of their overtime working behavior is to accomplish expected wage savings.Under the constraint of expected wage surplus , wage surplus effectis more powerful than substitution effect, which make the labor supply curve downward sloping.Migrant workers’ overtime working sustain the unlimited supply of labor and strengthen the dual economic structure.This is a new annotation to Lewis dual economic structure theory.For the steep labor supply curve of migrant workers , labor supply hours reduce by the rapidly rising wages, the cost shock and labor supply shock will occur at the same time. Urbanization is a double-edged sword, it can increase aggregate demand, but also reduce the labor supply of migrant workers. We suggest to strengthen skill training of migrant workers, to accelerate the land circulation, to achieve scale managment of agriculture; to establish and perfect the social security system of rural areas, to establish the policies which is people-oriented and to share the achievements of economic development.