Abstract: Offsets and errors of the sub-reflector and feed of the 40m antenna are analyzed and calculated according to displacement data obtained by the mechanical analyses. From the point of view of the error compensation，the lateral offset eror compensation of the sub-reflector is discussed and calculated by applying the best re-pointing technique. The result shows that the lateral offset error of the sub-reflector is fallen distinctly by the compensation. These calculations and analyses provide theoretical bases for the 40m antenna engineering.