Single stage G-M cryocoolers (GMCs) with high cooling capacity at 20-50 K provide significant benefit for the field of high temperature superconductors, however, their working efficiency can still be improved for commercial applications. Numerical simulations can serve as a valuable guide for precise optimization of the GMC because they allow one to study its internal operating characteristics. In this research, a twodimensional, transient model of a single stage GMC with a mechanically driven displacer is built and studied using computational fluid dynamics (CFD) simulation. The modelling method has been tested and experimentally verified. It enables a view of the instantaneous non-uniform flow and temperature distributions inside the GMC. Additionally, the effects of cold end channels on the heat transfer efficiency of the cold heat exchanger while operating at a large cooling power are analyzed. The results show that during certain periods of a cycle, most of the regenerator near the cold end remains at almost the same low temperature with no temperature gradient due to the small heat capacity of the materials compared to that of the helium gas; while most of the pressure drop occurs in the region near the hot end due to the high viscosity of helium-4. For a good cooling performance at low temperatures, the cold end channels should be distributed uniformly to guarantee a uniform flow and temperature distribution in the regenerator; while for a high output cooling power at high temperatures, they should be designed to enhance the heat exchange between the gas and the cold heat exchanger as much as possible. (C) 2017 Elsevier Ltd. All rights reserved.