In order to eliminate the parasitic fringe error introduced by PBS adhesive layer in michelon’s measurement optical path, a dynamic interference optical path with TP (twin PBS) structure as the light splitting core is proposed. Starting from the Mach Zehnder optical path, the structure integrates the polarization optical requirements of synchronous phase-shifting, the requirements of open external measuring arm, the requirements of compactness and adjustment convenience, and the requirements that the test beam and reference beam are separated only on the coating surface in PBS. The polarized light transmission model is constructed by Jones matrix. Through the simulation of ZEMAX and other software, the compact design and adjustment process flow of the new optical path are designed, and the adjustment steps are compressed. Finally, the TP dynamic interferometer composed of light source module, imaging module, modular optical path and polarization CCD is used to pre measure the three mirror system before coating, and the precision measurement under (4%)3 extremely weak reflected light is realized.