The drillstring on the derrick and hook of float drilling platform will be up and down under the action of wave periodically，the drilling pressure will be changed. In order to reduce the effect，the drillstring heave compensation system for offshore floating platform based on electrohydraulic proportional valve control asymmetric cylinder was designed. The analyses for electrohydraulic proportional valve and hydraulic cylinder were carried out，active and semi active compensation system simulation models were set up based on AMESim software and the PID control algorithm was simulated.The results show that the designed compensation system has good compensation effect.