A new real-time three-dimensional (3D) measuring method is proposed based on single-shot four-grayscale fringe projection, in which different non-zero three-scale grays in the four gray levels are cyclically arranged in equal width to encode a frame of three-grayscale fringe pattern. When the three-grayscale fringe pattern is projected onto the measured object, the corresponding four-grayscale deformed pattern is captured by the imaging camera, and the fourth grayscale corresponds to the shadow area in the deformed pattern. By the image segmentation method, non-zero three-grayscale fringes in the four-grayscale deformed pattern can be extracted respectively. After binarization, three frames of binary deformed patterns with 1/3 duty cycle and relative displacement of 1/3 period can be demodulated. So three frames of sinusoidal fringes with a 2π/3 shifted phase can be extracted by the inverse Fourier transform after filtering the corresponding fundamental frequency of the three frames of binary deformed patterns, and the 3D shape of the object can be reconstructed by phase measuring profilometry (PMP). The experimental results confirm the feasibility of the proposed method. Because of the fast refreshing rate and the insensitivity to the gamma effect of the projector, the proposed method has a potential application prospect in real-time 3D measurement.