The invention discloses a watertightness detection device and a hydraulic system thereof, and belongs to the field of marine equipment. According to the process, a water path between a first hydraulicpump and a detection part is controlled through a switch module, when the switch module connects the water path between the first hydraulic pump and the detection part, the first hydraulic pump conveys water to the detection part, and after the pressure in the detection part reaches designated pressure, the switch module disconnects the water path between a water tank and the detection part. At the moment, the detection part begins to be subject to timing, and after pressure maintaining of the detection part is carried out for the designated time, a first two-position two-way reversing valveis opened, the detection part is subject to pressure relief, and water in the part partly flows back to the water tank. In the process, the hydraulic system achieves automatic water filling and pressure relief of the detection part, the pressure in the detection part is automatically recorded, the detection process can be completely carried out through a program, manual participation is not needed, and part watertightness detection efficiency and detection result reliability are improved.