This paper presents a simple method for flat mirror tilt and piston measurement. The method only needs an LCD screen and a CCD camera. LCD screen displays structured light pattern, and camera takes the pattern's virtual image (VI) reflected by the mirror. Pose relationship between camera coordinate system and VI coordinate system can be calculated by camera calibration. Through coordinate transition, the relative tilt and piston of the mirror can be obtained. This method is simple in both structure and principle, and experiments prove that it can measure the flat mirror tilt for about 1″ accuracy and piston for less than 1um accuracy. And under the guidance of our method, a mirror can be adjusted to a specified pose.