An in-situ calibration method based on the autocollimator is proposed. Based on the principle of circumferential closure and the nature of Fourier series, this paper establishes the functional relationship between the measured value and the ideal value. Using the deviation between the ideal scribing position and the actual scribing position, we obtain the calibration curves. The calibration principle is deduced and analyzed in detail. The calibration angle measurement system is built and verified by experiments. The experimental results show that the original angle measurement error of the single readhead angle measurement sensor is 734.8″, and the error after calibration is 2.4″, and the calibration effect is better than that of the commonly used harmonic compensation method. The repeatability of the calibration system is better than 0.13″. The in-situ calibration method can effectively reduce the angle measurement error, and the method is simple and the calibration efficiency is high.