The invention discloses a hydraulically-controlled automatic accelerator. The hydraulically-controlled automatic accelerator is applied to accelerator control of hydraulic engineering machinery and consists of a hydraulic control valve and an accelerator adjustment mechanism, wherein the hydraulic control valve consists of a proportional pressure reducing valve and a hydraulic cylinder with two extension bars; the pressure change of a system is converted into a variation of a linear travel of a part by the hydraulic control valve, and the travel acts on an accelerator cable through the accelerator adjustment mechanism, so that the power output of the engine is matched with power required by the system. According to the accelerator, an aim of automatically controlling an accelerator is fulfilled, the working condition of the hydraulic system is tightly contacted with the control quantity of the accelerator, an operator does not need to manually operate the accelerator frequently, a phenomenon of engine stalling due to misoperation of the accelerator is avoided, and simultaneously, an aim of saving fuel oil is fulfilled; and therefore, the accelerator has the effect of killing two birds with one stone.