Overflow valve and hydraulic system

The invention discloses an overflow valve and a hydraulic system. The overflow valve comprises a valve body, a nut, a screw, a spring seat, a valve element, a first spring and a second spring. Wherein the valve body comprises a liquid inlet, a liquid outlet and a valve cavity, the valve element and the spring seat are located in the valve cavity, and the valve element abuts against the valve body at one end of the valve cavity; at the other end of the valve cavity, the valve body is provided with internal threads, the nut is provided with internal threads and external threads, the external threads of the nut are connected with the internal threads of the valve body, the internal threads of the nut are connected with the screw, the bottom of the screw abuts against the top of the spring seat, and the position of the spring seat can be changed by adjusting the screwing depth of the screw; the opening pressure of the overflow valve can be controlled; a valve element boss is arranged on the valve element, one end