The use of high arsenic groundwater for irrigation has been prevailed for decades in the Datong Basin. To reveal the characteristics of As mobilization in topsoil under irrigation activities, a filed plot-experiment has been conducted in the central area of Datong Basin. The irrigation activities promoted Fe(II) oxidizing to Fe(III) (oxyhydr)oxide, and As was adsorbed on Fe(III) (oxyhydr)oxide. As a result, the contents of As and Fe in sediments increased finally. Meanwhile, the irrigation activities led to the increase of soil salinity. Moreover, inverse geochemical modeling suggested that geochemical process was conducted during field experiment. The results illustrated temporal changes in As concentration under irrigation activities.