Based on the pinhole camera model,a geometry calibration method for circular trajectory scanning in cone-beam CT was proposed to reduce the geometric artifact of a reconstructed image caused by system errors. Firstly, the properties of the projected ellipses on the detector generated by the coaxial rotating steel balls were used to deduce circular points. Then,the constraint equations of absolute conic were established based on the polar constraint condition and the intrinsic parameters were obtained by linearly calculation. Finally, the extrinsic parameters can be figured out with the geometry method and ellipse parameters based on the intrinsic parameters. Experimental results indicate that the relative precision of intrinsic parameters and the extrinsic parameters are respectively 0.193% and 0.2% while using this method to calibrate the cone-beam CT system. It concludes that the proposed calibration process is able to solve all distortion parameters, build geometric model and eliminate the artifacts caused by the misaligned geometry on the reconstruction images.The method is characterized by simpler modelling, stronger application ability, and could be used in all circular orbit CTs.