A flow control system for a hydraulic pump of a construction machine that can variably control a discharge flow rate of the hydraulic pump in accordance with load pressure generated by a hydraulic actuator. According to the system, the discharge pressure of the hydraulic pump and the operation rate of the operation lever for the hydraulic actuators is detected, and standard load pressures of the respective hydraulic actuators are set. If the discharge pressure of the hydraulic pump is higher than preset load pressures of the hydraulic actuators, the discharge flow rate of the hydraulic pump is reduced in proportion to the operation rate in accordance with degrees of loads generated by the hydraulic actuators. If the discharge pressure of the hydraulic pump is lower than the standard load pressures of the hydraulic actuators, the discharge flow rate of the hydraulic pump is controlled in proportion to the operation rate.