Child trafficking and informal adoption are major types of irregular migration of children in China. Based on the data obtained from the public welfare platform, this paper examines the spatio-temporal differentiation pattern and the comprehensive mechanism of the two types of irregular migration. The results indicate that there are more males than females in child trafficking but less males in informal adoption. The trafficked children are mainly 1-6 years old, while the adopted children are largely under 1. The irregular migration has occurred mainly among rural families, and presented a temporal trend of “high in the middle and low in both ends” over the period under study, with the highest rates in Sichuan and Henan Provinces. Affected by the gradient force of regional differences, the irregular migration of Chinese children is intervened by the other “two parties” to carry out a “three places” flow mode, thereby leading to a passive and irregular migration.