The effect of system parameters on the reconstructed image quality in in-line digital holographic microscopy was studied. Firstly the spatial frequency of object wave was computed. Under the assumption of perfect imaging, the spatial frequency of object wave in the image space was completely determined by the frequency of the surface structure of the measured specimen and the magnification of the imaging system. Then, a suitable magnification must be designed so that the spatial frequency of object wave in the image space could satisfy the sampling capacity, based on the analysis that all the spatial frequency components of object wave should arrive at the recording plane and be effectively recorded in order to acquire the reconstructed image with high quality. And the recording distance affects on the object size which can be effectively recorded. When the magnification was the same, the size became larger with the decrease of the recording distance. Both the simulation and optical experiments validate the correctness of the proposed analysis.