In this research, the numerical simulation of thermal and flow fields in a four-cylinder double-acting α-Type Stirling engine is carried out. The research objective is to study the engine internal thermal and flow fields as well as to carry out parametric analysis for varying rotational speed, charged mass, heating temperature and porosity in order to determine the best design of the engine. The engine working fluid is modelled as 3D domain with moving boundary on both sides of piston using the CAD software, CATIA. The model is meshed using the mesh module in ANSYS Workbench. The numerical simulation is then conducted in ANSYS Fluent. In this study, the regenerator channel of the engine is modelled as porous medium in the form of wire mesh. This research provides a tool for visualizing the physical phenomenon of working fluid in double-acting α-type Stirling engine, including its temperature, pressure and velocity vector field. Based on the simulation results, the engine performance includes power output and thermal efficiency under different operating condition is predicted. The corresponding engine speed to the engine maximum power output can be obtained.