Aiming at the paper flatness measurement, a method based on four-step phase-shifting algorithm has been studied and proposed for the structured light measurement. The sine light that projected to the paper surface by DLP is scanned in steps of 1/4 period. After modulated by the surface topography of the paper to be measured, the structured light distribution information is acquired by the camera along with the phase main value of the structured light obtained through four-step phase-shifting algorithm. Finally, the absolute phase demodulation can be achieved through the multi-frequency heterodyne principle and the paper surface topography can be reconstructed through the height information after mapping the relationship between the phase and height. Using paper surface coordinate point cloud height range and standard deviation for quantitative evaluation of its flatness, the experimental results show that the measurement accuracy and stability meet the requirement of paper flatness measurement with the accuracy of 0.1mm and the measurement time of less than two seconds.