Flatness is a crucial parameter describing geometry of the steel strip surface. Measurement of flatness is very important for control of strip shapes. In the paper, the phase shift algorithm was introduced in measurement of flatness, and a new flatness measuring system based on the phase shift technique was developed. A new formula was derived to compute phase value from one single fringe image, which made higher spatial resolution. The system can avoid adverse effects caused by motion of the strip. Compared with present flatness measuring systems, the system presented in the paper gained more accurate results of flatness measurement.