To reduce the pressure shock of the electro-hydraulic-servo piston hydraulic transformer with swash plate, to lower the noise and to improve the operational stability, this paper develops buffer slot in the valve plate of the hydraulic transformer based-on the fluid-structure coupling function of the software ADINA. In software Pro/E, fluid model and solid model of the flow assignment structure of valve plate are built and imported to the software ADINA for building of finite element model. The flow field of the flow assignment structure that was checked with finite element method was analyzed. A valve plate with buffer slot with triangle shape was manufactured and tested on hydraulic transformer test-bed. The results revealed that the valve plate enabled hydraulic transformer steady in operation and the minimum speed of hydraulic transformer reached 35 r/min. The noise was lower and the reliability was improved. The correctness of theoretical analysis was testified.