Focusing and leveling measurement system(FLMS),as a key subsystem of the scanner,is used to detects height and tilts between wafer exposure area and the projection lens in real time. Its performance greatly affects the image quality of projection lens. Based on the grazing-incident beam method ,FLMS detects height and tilts by using more than 3 probe spots on wafer exposure area. Spot position errors cause measurement errors and defocus of exposure area which affect the image quality. Based on the one-spot measurement model,position error model of probe spot is presented. It is found that the height measurement error is proportional to the spot position error,and the accuracy caused by spot position error is less than 10 nm when the zero plane of FLMS is well adjusted,and spot position error is 0.05 mm.