A new phase unwrapping method based on dual-frequency fringe is proposed to improve both high accuracy and large measurement range of three-dimensional shape measurement by synthesizing the projected dual-frequency fringes obtaining higher and lower frequencies. The lower-frequency one is their phase difference, which can help unwrap the wrapped phase of the higher-frequency one from their phase sum. In addition, the relationship between the measuring accuracy and the frequencies of the projected fringes is studied to guide the frequency selection in actual measurement. It is found that the closer the two frequencies are, the higher the measurement accuracy will be. The computer simulation and experiment results show the viability of this method.