This paper analyzed the structure of the circuit and its overflow valve in the throttle governing circuit experiment under which the setting pressure accuracy of pilot overflow vale is low. The solution is substituted the way of inner leakage of pilot overflow valve with outer one. At the meantime the opening and closing characteristics of overflow valve of both were tested. Experimental results show that the static performance of the outer leak type and its setting pressure accuracy of the pilot overflow valve can be significantly improved. It has guiding significance for production practice and provided a simple and practical method to improve the performance of the equipment.