Due to the influence of current sampling and current reconstruction deviation, non-linearity of the inverters and parameter variation of the motors, DC offset component and high-order harmonic components exist in flux model, which makes the estimation of traditional rotor flux observer inaccurate and even divergent. In this paper, a rotor flux observer of permanent magnet synchronous motors (PMSM) with adaptive flux compensation is proposed. The DC offset component and the high-order harmonic components with the flux integrator observer can be eliminated by fed them back to the input of the flux integrator through PI regulator, so that the output of the flux integrator observer only contains the fundamental component, thus the rotor flux can be accurately estimated. Since the parameters of PI regulator are related with the estimated motor angular velocity, the control system has good robustness in a wide speed range. The flux observer is based on the voltage model in stationary coordinate system, so it is suitable for both surface and interior PMSMs. Moreover, the control method of the whole system has less computational complexity and fewer variables, it is easy to implement on a low-cost hardware platform. Finally, the experimental results verify that the flux observer has superior performance.