Off-axis deflectometry is widely applied in the measurement of specular surfaces. However, the measuring accuracy depends on the reliability of geometrical calibration. Existing methods are inconvenient to be utilized due to their disadvantages of low efficiency and operational complexity. A simple geometrical calibration method is proposed by applying a flat mirror with markers, and only one image needs to be captured. A compensation process is introduced to correct the form error of the mirror. Experimental results show that the re-projection errors decrease from 0.319 pixels down to 0.12 pixels; thus the measuring efficiency and accuracy of optical surfaces can be greatly improved.