A new algorithm insensitive to the piston and tilt errors of reference mirror for phase stepping interferometer is presented. Clearly

if the reference mirror has tilt and piston errors caused by phase-shifting

every pixel on the same interferogram will have different phase-shift values. And by the fact that the phase shifts of all the pixel on the same interferogram are still kept on a same plane

so by least-squares fitting to the phase-shift plane

the errors can be eliminated. The validity of this algorithm has been demonstrated by means of computer simulation.