The jet pulsed assembly is the main component of the jet type hydrodynamic oscillator. The stable reversing of the piston rod is the key to the normal operation of the jet type hydrodynamic oscillator. In order to solve the problem of commutation instability of piston rod, the dynamic characteristics of the reversing process and principle of piston and the piston rod at the upper and lower dead points are studied, the piston rod and throttle disc structure are optimized. The results show that when the jet pulse piston is located at the lower dead point, the flow area of the throttle disc is the smallest and the reversing distance of the piston is the largest, which seriously hinders the stable reversing of the piston and piston rod. Then the structure of piston rod and throttle plate is improved, which basically eliminates the commutating resistance when the piston is located at the lower dead point, and improves the stability of the jet pulsed sub. Finally, the reliability of piston commutation optimization is verified by ground test, and it is found that the new structure can reduce the tool pressure drop and increase the pressure wave amplitude. This paper reveals the reason of the instability of the jet pulse sub commutation, optimizes the commutation structure design of the piston, and improves the reliability of the piston commutation, which is of great significance to the field promotion of the jet hydraulic oscillator.