In order to solve the online calibration problem of the angle measuring system in practical application, an online self-calibration system of circular grating angle sensor was designed. Based on the principle of round closure and Fourier series, the relationship between the measured value and the error of single reading head was established, and the single reading head self-calibration was realized. The self-calibration system consisted of a reading head, a glass circle grating, a single-axis turntable and a multi-channel acquisition control system. The experimental result shows that the accuracy of the single reading head of the self-calibration system is ±5.90″, which is close to the accuracy of the harmonic compensation method with external reference standard, the measuring repeatability is less than 0.76″. Self-calibrated measurement system could realize the online calibration of circular grating angle measuring system, which meets the requirement of precision and reliability in precision measurement field.