The grating projection measurement method has broad applications in surface 3D topography measuring due to its measurement speed and accuracy. Along with the grating, the phase-shift method is usually adopted for calculating the field phase. This is achieved by projection scanning in order to obtain more grating fringe images. The higher the projection fringe density is, the higher resolution can be achieved. However, because the results of the projected grey value periodically change, once the fringe period is over the single-period, the absolute-phase will become wrapped-phase. Always obtain the absolute phase by means of an unwrapping algorithm because the traditional projection unwrapping algorithm, which is based on phase continuity and changing conditions, it is not suitable for step-height measuring. Aiming to solve this problem, an unusual sub-step projection scanning method for variable widths of grating periods is proposed. According to the principle of minimum error, using the step-by-step phase estimation and connection method, we can directly determine the high-density fringe projection absolute-phase.