In order to obtain accurate power factor in user side and to reward and penalize users reasonably in power grid containing large amounts of harmonics, it is necessary to improve the power factor defined in the method of power factor adjustment tariff. And an accurate model of power factor measurement is proposed. In this paper, the rationality of power factor defined in the method of electricity charge under harmonic background is analyzed. First of all, it is pointed out that the power factor of the users with linear passive inductive load is too small in the harmonic systems, it means that linear users are not only subject to harmonic damage, but also to pay more electricity. Then, simulation is carried out to validate it. Finally, according to the principle of the division of harmonic responsibility and the influence of harmonics, a new power factor measurement model based on IEEE.Std 1459-2010 is given. This model can not only provide some reference to measure reasonably and fairly for the power supply department, but also provide an important reference for the assessment system.