The invention provides a control method for a hydraulic system of an engineering machine, and belongs to the technical field of hydraulic pressure and control. The control method comprises the following steps: an electronic control execution module judges a working mode of an executor according to a system input signal; a pressure detection module detects exit pressure of a hydraulic pump, and an engine speed sensor detects a speed of an engine; and the electronic control execution module compares the exit pressure with preset pressure according to the judged working mode and compares the speed of the engine and the engine speed in such mode, and under the condition that the exit pressure does not meet preset pressure, the electronic control execution module controls the opening of an electromagnetic valve to change the oil spraying quantity of an oil sprayer, then the speed of the engine is changed and thus power transmitted to the hydraulic pump by the engine is changed. According to the control method, by controlling the displacement of the hydraulic pump, the exit pressure of the hydraulic system can quickly and efficiently reach the preset pressure of the hydraulic system, so that the system power is saved, and the working stability, the accuracy, the flexibility and the working efficiency of the hydraulic system are improved.