To implement the in-line self-calibration of a time grating angular displacement sensor in practical applications, a self-calibration method called Fixed Angle Shift (FAS) was presented and a self-calibration system was designed. Firstly, circle-closed natural base was shifted to a fixed angle base to be as the self-calibration base of time grating. According to properties of Fourier series, the relations between differences of measured values and errors were established by shifting the fixed angle base to amplitudes and phases in the Fourier transfer. Then, the error function was reconstructed based on the Fourier analysis for differences of measured values. Finally, the error sources effect on the calibration accuracy were discussed and a zero point correction algorithm was designed. To verify the results of self-calibration, self-calibration experiments were performed and compared with the proposed FAS system and a laser interferometer. Experimental results indicate that the accuracy of self-calibration with FAS method is 1.9″, which is well consistent with that of the theoretic calculation (1.5±0.5)″. The method solves the self-calibration base of the time grating angular displacement sensor and satisfies, the requirements of precise measurement systems for high precise and high reliability.