Based on the Strehl criterion of coherent lens imaging system, the focal depth (FD) of lensless Fourier digital micro-holographic system is deduced. The FD expressions of the holographic systems with symmetrical and unsymmetrical reference point sources are obtained. The results show that the FD of lensless Fourier digital micro-holographic system is dependent not only on the optical wavelength and the numerical aperture of the recording system but also on the offset of the reference optical source, which is different from the FD of lensless Fourier digital micro-holographic system. The validities of the theoretical analysis are demonstrated by the computer simulations and experiments.