In traditional Gabor in-line holography, direct passing reference wave and diffracted object wave overlap in space, thus it is unable to realize phase-shifting interferometry. A method to realize Gabor in-line phase-shifting digital holography is presented. Based on the characteristic that different spatial frequency components are separate in the spatial frequency domain, a spatial light modulator is placed in the spatial spectrum plane of the 4f system to introduce phase-shift to the zero-frequency component of spatial frequency spectrum, and the corresponding phase-shifting algorithm is utilized to obtain the reconstructed image. Theoretical analysis and experimental results show that the proposed method can improve the quality of reconstructed image while retaining the original advantages of conventional Gabor in-line holography at the same time.