Non-isothermal forging has been widely employed in the metalworking industry. Although the die-chilling effect is very significant in the non-isothermal forging process, the process parameters of time and temperature are difficult to measure and control during non-isothermal forging. To cope with this problem, this study employs the thermal-coupled finite element method to examine and to establish the relationship between the process parameters and the deformation behaviour, then deriving useful design rules. The detailed process model based on experimental conditions, and the flow stress model constructed by the localized linear fitting and interpolation method, were used in the finite element analysis. Consequently, the simulation of the non-isothermal forging of Ti-6Al-4V was performed, the predicted results agreeing well with those from experiment. Due to the large temperature gradient, the mode of deformation in non-isothermal forging is different from that in isothermal deformation. Therefore, a deformation mechanism for non-isothermal forging was proposed in this paper to explain the deformation behaviour. Hence, the deformation mechanism governing the metal flow of non-isothermal forging can provide guidelines for forging die design and process design.