The present disclosure relates to an automatic transmission control unit for construction machinery and a control method therefor, and when a load pressure detecting unit detecting a load pressure of hydraulic pressure supplied from a main hydraulic pump to an actuator, such as a cylinder and a hydraulic motor, is in an abnormal state, the automatic transmission control unit receives an electronic control pressure signal having little concerns of a breakdown or a malfunction, and automatically changes a travel speed of construction machinery based on the received electronic control pressure signal.