In order to improve the power system phase and power measurement accuracy when frequency offset, this paper presents a modified DFT of phasor and power algorithm based on improved expansion Kalman filter (IEKF) frequency measurement, the measurement error of DFT under nonsynchronous sampling caused by frequency offset is analyzed, and the function relationship between the phase, amplitude and frequency offset and initial phase angle is established. The frequency offset is obtained by IEKF, and then the DFT calculation results can be corrected to obtain the real phase and power of the signal. The simulation results show that the proposed method compared with the traditional adaptive DFT algorithm can effectively eliminate or weaken the influence of harmonics, noise and frequency offset on the synchronized phasor measurement, and improve the precision of phasor and power measurement. This work is supported by National Natural Science Foundation of China (No. 51505317) and Natural Science Foundation of Shanxi Province (No. 2015011057).