Abstract: The hydraulic control system of the underwater manipulator introduced in thisarticle is a typical,closed,changable return pressure system,This article explainesthe design principle of the hydraulic driving mechanism and the analyzing and calcu-lating methods of static and dynamic parameters,and it gives a detailed analysisof the static characteristics of unsymmetrical cylinder controled by four-way valveat changable retum pressure and also gives the law of variation of the valve's maxi-mum no-load flow and load pressure drop at different return pressure and at differ-ent area ratio of the cylinder.The article provides bases for determination of theparameters on the driving mechanism of hydraulic control system of the underwatermanipulator.