A position sensitive detector(PSD) was adopted to collect the pendulous signal from the measured elastic device.Then the collected signal is sent to a computer,and the FFT analysis of the signal is completed there.The pendulous frequency of the measured elastic device can be obtained.from the related FFT diagram.The system possesses following advantages:the measurement is non-contacted one;the measuring response is quick;and the system is easy to be operated.Experimental results show that the measurement resolution of the frequency is 0.01 Hz and the repetition of the measurement results is 0.03 Hz.