The paper designs a pulsed plasma thruster system based on water propellant and studies energetic threshold of its main working characteristics to solve the problems of low performance and high pollution of Teflon pulsed plasma thrusters. Based on an analysis of the forming condition and cause of electric discharge types, working energetic threshold is studied experimentally by the discharge voltage and current measurement.The study shows that the initial plasma that can provide trigger probability for steady operation and energy storage capacity are obtained only by enough electric-field intensity and trigger current, and the normal working energy range of the water propellant pulsed plasma thruster is decided by stored energies but not related to capacity values. The study provides a possibility for developing low power water propellant pulsed plasma thrusters of free pollution, high performance and wide applications.