To reduce the flatness measurement errors of load motion trajectory in spatial t arget simulator and to improve the flatness precision,the error sources are qua ntitatively analyzed by physical modeling and numerical calculation,and a new me thod is proposed for building weights according to the test point coordinates′ residues.The weighted least squares method is used to fit the best plane,and the point-surface distance and dihedral angle are used as the flatne ss evaluation index.In a non-contact measurement experiment with laser tracker as a tool,the simulator load′s flatness is calibrated with this method,flatness precision reaches 0.4mm,and dihedral angle is an order of magnitude lower than that in the non-weighted method.The results show that in the range space of 3m×2m,this calibration and experiment method is able to ensure the flatness measurem ent p recision.