Based on the panel data of 29 provinces and municipalities in China from 1992 to 2017, this paper systematically analyzes the relationship among transportation infrastructure construction, labor mobility and urban-rural income gap by applying the nonlinear threshold model. The empirical results indicate that transportation infrastructure construction can effectively promote the flow of agricultural labor to non-agricultural sectors, which affects the level of urban-rural income gap. What's more, the impact of transportation infrastructure construction on the urban-rural income gap is not a simple linear correlation, but a U-shaped dynamic relationship with the change in the degree of rural labor transfer. If the transfer rate of rural labor is lower than 0.157, the construction of transportation infrastructure can narrow the urban-rural income gap. If this rate exceeds 0.288, the transportation infrastructure construction is not conducive to narrowing the urban-rural income gap. Moreover, as this rate further rises, the negative impact of transportation infrastructure construction on the urban-rural income gap will be greater.