Ronchi shearing interferometry modulates the spatial coherence of light field with extended source. With the advantages of simple structure, common- path, null- fringe detection, and so on the Ronchi shearing interferometer can be used for in situ aberration measurement of lithography projection lens. Ronchi shearing interferometry for wave-front aberration measurement of lithography projection lens is studied. The interference model of Ronchi shearing interferometer is derived by theories of grating diffraction and spatial coherence. A tenstep phase-shifting algorithm is proposed to eliminate negative effects of unwanted interference from the high order diffraction light, which limits the accuracy of phase retrieval. The effects of the first ±9 orders interference on phase retrieval accuracy can be eliminated. The peak-valley (PV) value and root mean square (RMS) value of theoretical phase retrieval error are less than 0.0046 λ and 0.0019 λ respectively. The theoretical interference model and the phase retrieval algorithm are validated.