The machining error caused by the gravity of the ram was analyzed. A new type of error compensation mechanism was designed.Gyroscopes and acceleration transducer were used to measure the ram’s angle change in vertical direction,the signal was input computer through A/D conversion,then it was passed into the electrohydraulic servo valve and hydraulic cylinder through AT89S52 singlechip.The vertical direction’s error was compensated by vertical guide mechanism which was driven by hydraulic cylinder.The compensation method is lowcost, the machining error is controlled in an appropriate range.So the machining precision is improved.