Abstract: This paper discusses the application of digital control in electro-hydraulic Servo loading system.With a microcomputer and a A/D and D/A module,applying a open-closed loop optimized design method, the control in electro-hydraulic servo system is realized. Besides, some PID algorithm is studied. It interprets the system hardware,software,and experiment.On the basis of full throretical design,a lot of experimental researches are made.Experimental results of the two algorithm are analysed by comparison and in effect,the indexes of open-closed loop algorithm are higher than those of PID algorithm.Finally,this paper studied the extraneous force's elimination.Experimental results proved that the method of eliminating extraneous force is successful and effective.