It is significant for the safe and stable operation of power system to accurately diagnose and evaluate the insulation status of oil-paper-insulated power equipment. Frequency-domain dielectric spectroscopy has the advantages of abundant insulation information and strong anti-interference ability. However,it takes a lot of time for low-frequency test process of frequency domain spectroscopy (FDS),which has a negative influence on field testing,diagnostic and evaluation. In addition,the FDS curve is very vulnerable to external interference and result in test error due to the low current amplitude in the low frequency band. In order to solve this problem,a mixed solution of time domain method and frequency domain method is adopted. In the low frequency band,the Fourier Transform is used to transform the time domain polarized current signal into the frequency domain to obtain the loss factor of the low frequency band. Meanwhile,a set of insulation diagnosis devices is designed in this paper based on hybrid time and frequency domain method. The linear cascade amplifier has the advantages of high bandwidth,high voltage and low harmonic distortion. A symmetric differential structure is adopted in the micro current measurement circuit,which suppress temperature and power frequency interference effectively. Finally,the accuracy of the device is verified by the experimental test on the oil-paper insulation bushing model of the transformer.