Based on the theory of wavelet-transform analysis for spectral phase retrieval from spectral shearing interferogram, we extract phases from the traces of frequency resolved optical gating (FROG) with time-frequency analysis method. The method enables the phases to be extracted from the ridges of FROG traces directly, and requires no iterative algorithm. For the flap ridges, accurate phases can be extracted. However, for the frequency-varying ridges, the results have great error at the tails of the pulses, and the reasons for error generation are analyzed.