After implementation of a digital electro-hydraulic（DEH） control system， the effect of primary frequency regulation（PFR） is not ideal. The main problems are existence of oscillations and insufficient PFR capability. Thorough detailed studies of power-frequency regulation control mechanism， it is discovered that the dynamic PFR process at the power output control module demonstrates feed-forward open loop characteristics when subjected to abrupt disturbances. On the other hand， when the system is at steady state or leading regulate state by pre-set value， it illustrates closed-loop feedback characteristics. Feedforward open-loop appear“reverse regulation” in the beginning of the PFR. The simulation results prove the correctness of qualitative analysis and quantitative analysis. The analytical outcome provides important reference and basis for improving power-frequency regulation method.