Abstract: For monochromatic light interferometry under open-loop phase shifter, a high precision method for phase recognization was developed to satisfy four-step phase-shift algorithm. Firstly two pixels with suitable phase-difference were selected from the interference field, and the interference equations of the two pixels gray values were established in one phase-shifting cycle. Secondly, the interference equations parameters could be obtained by using ellipse fitting algorithm, and the point-to-point driven step length and sequence phases could be determined through back calculation of sequence phases. Then four interference gray scales meeting the four-step phase-shift algorithm were designed and calculated through Lagrange parabolic interpolation, which were used to calculate the initial phase of every pixel. Finally, the surface topography was calculated through multi-wavelength algorithm and the measurement errors were analyzed. Experimental results show that the Ra value of square wave specimen with multiple grooves is 0.4390m, and the measurement error is 0.23%. This method decreases the requirement for hardware and environment. It can meet the high precision demands of surface topography measurement.