Abstract: In conventional radiotherapy,the images obtained by the image intensifier of radiotherapy simulator are often distorted.In order to ensure the accuracy of radiotherapy,corrections must be conducted.Hence,an image correction algorithm is proposed with the objective function based on the distance of a point to a straight line.Given the correction speed,precision and smoothness,the moving least square method based on the primary linear base is adopted to correct images firstly and the results are satisfactory.Furthermore,on account of the characteristic that corrected grid lines should be straight,the number of reference points is increased so that the middle area of intersection points has reference points too.The corrected results present that after the correction,grid lines are more similar to straight line,and the correction effects of intersection areas are better than those of correction algorithm,which only take into account the intersection points.In addition,although more than 2 000 reference points are increased,the algorithm running time remains short.