To solve the problem of the measurement error of phasor and electrical parameters in the power system when frequency offset, an improved DFT correction algorithm is proposed. The function relation between the real part, the imaginary part of the phase and the amplitude, the initial phase angle, the frequency offset is established. According to this relation expression, the real part, the imaginary part and the frequency offset satisfy an identities. Therefore, the correction algorithm of the amplitude and phase angle of the phase can be obtained under the known frequency. Further, the corrected real and imaginary parts of the phasor are derived. Finally, a correction algorithm for other electrical quantities is obtained. Simulation results show that when the system frequency is shifted, the algorithm can effectively improve the accuracy of phasor and electrical quantity when the signals are pure sinusoidal signals or the signals with higher harmonics and random noise. Meanwhile, the algorithm is insensitive to the sampling frequency and frequency error, and the algorithm has a small amount of calculation and does not increase the extra delay time.