One four redundant hydraulic actuator is a pivotal component of one aircrafts horizontal tail steering system. The output characteristic of normal state and three typical fault states for hydraulic actuator servo control systems with single channel and four redundancies was researched in detail. The simulation test and research were carried out. The simulation test results show that hydraulic actuator servo control systems can not implement tracking of all the input signals (anticipant output) effectively under the circumstances of three typical fault states. The research work can provide a massive base to understand the work principle, the work course and the output characteristic of the hydraulic actuator.