A new piezoelectric/ electrorheological hybrid hydraulic ultra-precision step actuator that uses a hybrid mode of piezoelectric stack pump hydro-drive and electrorheological valve control to the precision hydraulic cylinder is presented. It achieves high frequency, ultra-precision hydraulic step drive. The working principle of the step actuator is analyzed and its experimental characteristics are tested. Experiments show that the piezoelectric/ electrorheological cooperate ultra-precision hydraulic step actuator provides new theory and method to the development of precision step actuator with high resolution and large remove, high-speed, big load ability, it provide a basic research reference in the field of ultra-precision drive technology.