X-ray fluorescence computed tomography (CT) based on an X-ray tubes is affected by many factors, which thus results in poor image quality. Geometric parameter error is one of the important factors that restrict its high-quality image reconstruction. In this paper, the influence of the relative position deviation between the two-dimensional planar phantom and the detector on the projection data is analyzed, and the correction of geometric deviation parameters is realized with the local linear relationship between the original projection data and the reprojection data of the original reconstructed image. Geant4 is employed to simulate the fan-beam X-ray fluorescence CT system with deviation parameters, and simulated projection data are used to verify the correction method. The results show that the method can accurately calculate the geometric offset to a large extent and effectively eliminate the influence of geometric parameter error on the reconstructed image. Regarding the reconstructed image after correction, the information entropy is reduced and the average gradient and standard deviation are enhanced, with the image quality thus improved.