Depth of focal (DOF) is an important parameter characterizing the performance of imaging system. Based on the criterion of the DOF of coherent lens imaging and by using numerical integration and computer simulations, the DOF of Fresnel digital holographic imaging system is deduced and analyzed. The corresponding expressions are compared with that obtained by the phase error criterion of ideal imaging. The results show that the DOF of Fresnel digital holographic system is different from that of coherent lens imaging system. With the definite recording distance and definite parameters of CCD, the DOF of off-axis Fresnel system is slightly larger than that of in-line system. Furthermore, the DOF of off-axis system with symmetrical reference offset is larger than that of the unsymmetrical one a little. The DOF obtained by image intensity criterion can be better characterized the performance of digital holographic imaging system.