Based on therotical analysis and experimental demonstration,three commonly used reconstruction algorithms in digital holographic microscopy,Fresnel transform algorithm,angular spectrum algorithm,and convolution algorithm,are investigated. It shows that the lensless Fourier transform digital hologram can be reconstructed at any distance by Fresnel transform algorithm. For convolution algorithm,the whole reconstructed image with high resolution can be acquired in an optimal reconstruction range or a small area around it. When the reconstruction distance is slightly smaller or larger than the optimal one,a high quality image can also be obtained by using angular spectrum algorithm. Angular spectrum algorithm is better than convolution algorithm as a whole. The Fresnel transform algorithm is time-efficient and simpler than the other two.