Theoretical defects of the Least Squares Method(LSM) in the evaluation of the spatial straightness error was analyzed in detail with geometry,the theory of errors,and optimization principle so as to effectively boost the precision of the assessment of the spatial straightness error.The mathematical model of improved LSM algorithm was proposed after the LSM was modified.Finally,the improved LSM algorithm was programmed,and the corresponding numerical experiments were done.The experiment results have shown th...