In the study of hydroelectric system， the precision mismatch between the hydraulic system and the power system models， the lack of a hydraulic-mechanical-electrical system model impede the transient process study and improvement of simulation precision on the hydroelectric system. Based on the individual component model， an integral hydraulic-mechanical-electric system model is established using MATLAB/Simulink modulars. This model suits to study the interaction between the hydraulic system and the electric system， and analyze the transients of a hydro plant. The interaction between the power system oscillation and hydraulic system oscillation is studied on a single machine infinite system.