Isothermal extrusion is a very much desired technology. However, its implementation in the light-metal extrusion practice has, up till now, been technologically constrained. In an attempt to realise isothermal extrusion, a simulation model based on the PID control algorithms was developed to establish ram speed profiles that could prevent extrudate temperature from further increase after an initial rise during an extrusion cycle. With this simulation model, extrusion ram speed could be adjusted in real time according to the simulated exit temperature. A case study was conducted on the simulated extrusion of a magnesium alloy AZ31B into a hollow profile. The results showed significantly improved temperature homogeneity not only along the extrudate length but also on its cross section in the case of extrusion in the isothermal mode with a designed ram speed profile. In addition, die temperature varied over a narrower range and the force acting on the die face was more stable over the process cycle, in comparison with extrusion in the conventional iso-speed mode.