Real-time calculation of power angle is usually used in power system stability analysis and oscillation identification, but the existing line power angle calculation method has a large error when the potential amplitudes on both sides are not equal. For this reason, a new real-time power angle calculation method is proposed. Two situations where the lowest point of the calculated voltage is located inside and outside the potential connection are discussed, and the power angle calculation formulas under different conditions are obtained and distinguished. The steady-state bus voltage is used to approximate the potential amplitude. This will bring a certain error to the power angle calculation. Therefore, the potential amplitude is corrected and the corrected power angle calculation error is compared with the error of the existing method. Simulation verifications based on PSCAD prove the reliability and effectiveness of the new method. The results show that compared with the existing power angle calculation method, the new method effectively reduces the power angle calculation error when the potential amplitudes on both sides are not equal and has good calculation accuracy and practical effect. This work is supported by the National Key Research and Development Program of China (No. 2021YFB2401000).