In order to diagnose the fault types of rolling bearings under variable speed conditions and improve the safety of equipment,by using the theoretical framework of synchrosqueezing transform (FSST) based on short time Fourier transform (STFT),a demodulated short time Fourier transform (DSTFT) was proposed.It was designed to solve the problems of inaccurate estimation of instantaneous frequency (IF) and time-frequency ambiguity caused by modulation component of STFT.A demodulation operator was introduced into STFT to eliminate the influence of modulation on the signal,and the more accurate IF estimation was obtained.On this basis,demodulation synchrosqueezing transform (DSST) based on DSTFT was deduced,by which highly concentrated time-frequency representations can be obtained through the rearrangement of time-frequency transformation coefficients.The proposed DSST method has the advantages of accurate IF estimation and good energy concentration of time-frequency ridge.The effectiveness of the proposed method in fault diagnosis of variable speed rolling bearings is verified by using numerical simulation analysis and vibration signal analysis of experimental bench.