Aimed at the problem that the output pressure of newly developed rotary direct drive electrohydraulic pressure control servo valve (RDDPV) was oscillating, the mathematical model and block diagram of RDDPV were established, and the stability criteria was obtained. In RDDPV, the electrical feedback of motor rotational angle and output pressure are used instead of traditional mechanical or hydraulic feedback. When the spool valve near the middle position between inlet and outlet port, steady state flow forces provides the positive feedback on spool motion, leads to a negative stiffness of mechanical and hydraulic parts, and the stability of RDDPV is poor. In order to improve the stability of RDDPV, the electrical feedback stiffness of the servo valve was improved by increasing the electrical feedback coefficient of motor rotational angle. Numerical simulation and experiment results shown that the stability of RDDPV can be effectively improved by increasing the electrical feedback coefficient of motor rotational angle.