Based on the method of three-flat testing, the comparison of flatness measurements by two kinds of instruments which are isoclinic interferometer and phase shift interferometer was given. With the study of the principles of two kinds of interferometers, a standard process was put forward to extract a few figures from the lattice data tested by phase shift interferometer to fit the data format from isoclinic interferometer. Compared to the structure and environmental control method of isoclinic interferometer, the temperature, temperature gradient and temperature stratification during the absolute measurement of flatness using the phase-shifting interferometer were researched, and the double insulation structure of the cavity of phase-shifting interferometer was applied. The measurement repeatability was 0.002 um, and the uncertainty of absolute measurement was 0.01 um. , The difference from the measurement result of isoclinic interferometer was less than 0.01 um，which illustrates the feasibility of the phase-shifting interferometer used for the flatness verification.