Voltage Controlled Oscillator (VCO) is an oscillating circuit whose input control voltage affects the output frequency and can be used to generate frequency modulation signals. VCO can be applied in security imaging, and the linearity of its output frequency determines the side lobe level and distance resolution of the imaging result. Therefore, a VCO open-loop linearity correction method based on polynomial fitting is proposed to improve the imaging effect of security inspection. The output frequency of VCO passes through a frequency divider, and the divided signal is filtered and then collected by AD. The nonlinear information of the VCO output is obtained through digital signal process in FPGA. Finally, VCO is corrected by the output voltage signal of DA. When the system transmits linear frequency modulated continuous wave (FMCW) pulse, this method is applied at the same time. So it has good real-time performance and it can solve the temperature drift problem of VCO so as to be applied for long-term synthetic aperture imaging. The feasibility and effectiveness of this method are verified by experimental results.