Based on recording the inputs and outputs of control systems and its control modules and control blocks during on-site disturbance experiments and tests, an abnormal signal analysis based method for locating oscillation source in control blocks is proposed by intensively comparing the differences between the ideal standard responses and the actual response of a control system and its control modules and control blocks. The method has a merit of being able to locate the power oscillation source to specific control blocks. By utilizing the proposed method, the oscillation source of power oscillation events occurred in a nuclear power plant is determined as the frequency measurement block in PSS of a digital excitation system.