To realize phase demodulation using a single deformed fringe pattern in 3D fringe sensing, a method is proposed. In the method, instantaneous frequency estimation of wavelet transform is applied to the deformed fringe pattern, and derivative frequency is used to directly extract the fundamental spectrum of the original fringe pattern from the instantaneous spectrum of its deformed fringe pattern. Firstly, theoretical analysis shows that if the instantaneous frequency at a certain position of the deformed fringe pattern is equal to the fundamental frequency of its original fringe pattern, the derivative frequency at such position is zero. Secondly, the instantaneous spectrum of the deformed fringe pattern is experimentally estimated by Gabor wavelet transform. Finally, the fundamental spectrum of the original fringe pattern is identified and obtained from the above instantaneous spectrum. It means that a single deformed fringe pattern is enough to realize phase demodulation. Experimental results of 3D shape measurement show the good effect of the proposed method in phase modulation.