The spring fastener that connects sleepers and rail in high-speed railway is with a high responsibility since its quality directly affects the safety of the railway system. This paper investigates the isothermal hot compression behaviour of a commercial medium carbon micro-alloyed spring steel (raw material) by a Gleeble 3500 thermal-mechanical simulator, which was used to produce spring fasteners. The dependency of the flow behaviour and microstructure evolution under different deformation conditions were systematically studied. The results revealed that the peak stress points are not necessarily recognised as the dynamic recrystallisation (DRX) behaviour because the partial occurrence of DRX may appear in flow stress curves with no stress peak. The metallurgical events that happen during the process of hot compression are close related to the phase transformation products. In the prior austenite grains, high dislocation density can promote the nucleation acicular ferrite. Nevertheless, the DRX behaviour consumed the stored energy, which is conducive to the transformation of bainite and martensite. Besides, the microstructure evoluted from prior austenite grains occupies a larger fraction of middle angle grain boundaries and higher mean local strain compared to the counterparts of DRX grains. The findings would provide useful references for the industrial optimisation in the hot forming process of a spring fastener using Si-Cr medium carbon spring steels.