Based on information theory, the relationship between the information content of digital holography and recording optical structure was studied. In-line Fourier transform holography which could realize the maximum information content recording was proved. Meanwhile, a new simplified phase-shifting digital holographic microscopy that can effectively eliminate the zero order and conjugate virtual image was proposed. In this technique, a quarter-wave plate was used as the phase-shifting device, and the original reference wave was simulated by computer. This technique just needs phase once, only two digital holograms and an intensity image of object wave. With the combination of the new phase-shifting technique and in-line lensless Fourier transform holography, the phase-shifting process is simplified and the reconstructed image is outputted by maximum pixels. Thus, the resolution of the reconstructed image is improved obviously.