This paper proposes an offline measurement method for a large aspheric mirror during its grinding stage. A measurement method and mathematical model to realize a three-dimensional (3D) profile measurement for large-diameter aspheric mirrors are proposed, based on a cylindrical coordinate system. The measurement error induced by the alignment error is examined based on the error analysis of the measurement and workpiece coordinate systems. The corresponding pose error obtained through the nonlinear least squares method is eliminated to improve the measurement accuracy. To evaluate the entire surface profile error, a 3D profile model is established. The total uncertainties of the proposed measurement system are also estimated. Comparison experiments for a 300-mm-diameter K9 glass workpiece are conducted on UPFM900 and Taylor Hobson PGI 3D profile instruments, and the profile accuracy is found to be improved from 15.3 to 7.12 μm after eccentricity and tilt error elimination.