The invention discloses a slightly-wetting irrigation water amount automatic control system which comprises a water tank. The water tank is connected with a water inlet main pipe and a water outlet main pipe. The water inlet main pipe is provided with a water inlet electric ball valve, so that water inlet flow of the water inlet main pipe is controlled. The water outlet main pipe is provided with a water outlet electric ball valve, so that water outlet flow of the water outlet main pipe is controlled. Water conveying branch pipes comprise two parallel pipes and are connected with the water outlet main pipe. A plurality of slightly-wetting irrigation pipes which are buried in soil in a paving mode are arranged between the water conveying branch pipes through a T-branch pipe. The water outlet main pipe is provided with a liquid flow sensor to sense water flow information. At least one soil water potential sensor is arranged in a cylindrical wetting body which is formed by extending in the length direction with the slightly-wetting irrigation pipes as axes to sense soil water potential values. A microcontroller controls the water outlet electric ball valve and the water inlet electric ball valve. According to the slightly-wetting irrigation water amount automatic control system, unattended operation can be achieved on a slightly-wetting irrigation process, control is automatic, the blindness of manual operation is effectively avoided, and continuous slightly-wetting irrigation is especially achieved on a farmland.