Hot forging processes of a spur gear shaft with three billet sizes are simulated using FEM. The billet with size of Phi 36*60 has the best formability in hot forging the spur gear shaft. The effects of the initial billet temperature and the friction coefficient on the hot forging formability of the spur gear shaft are obtained respectively through simulation. Hot forging experiments are carried out and experimental results show good agreement with those in simulation. At last, the reasonable hot forging parameters of the spur gear shaft for manufacturing engineering are suggested as follows: billet size of Phi 36*60, billet initial temperature of 1050 degrees C and friction coefficient of 0.3.