Underwater high power relay control is needed for many of the underwater engineering machineries which are equipped with high power motors.By a series of experiments,the influences of oil-immersing condition,water pressure,the kinds of oil,as well as operation procedures on the reliability and performance of an oil-immersed and pressure-compensated contactor were studied.It was concluded that an oil-immersed and pressure-compensated contactor with little current load can work well under deep sea high water pressure,but its arc-quenching performance becomes terribly bad if its current load is increased,resulting in its switching operation failure or even serious damage,so special operation procedures have to be taken for its possible use in deep sea conditions.Proper operation procedures have been given in the paper for the reliable use of deep sea oil-immersed and pressure-compensated contactor.The newly developed oil-immersed and pressure-compensated contactor has been successfully used during its sea trial in the Pacific Ocean.