Abstract: The hot deformation behavior of 22MnB5 steel was investigated through tension testing on a Gleeble 3500 thermal-mechanical simulator, over a range of temperature from 700℃ to 900℃ and a range of strain rate from 0.01 s^-1 to 10 s^-1. It is found that failure strain of the steel increases with the increase of strain rate, and this trend is intensified as the temperature rises. A unified viscoplastic constitutive model coupled with damage, based on dislocation density and incorporated effects of strain, temperature and strain rate, was established to mathematically describe the steep-fall stages of the stress-strain curves. Material constants in the model were determined and optimized by a genetic algorithm. The model can accurately predict the flow stress of the steel in hot stretch and can describe damage evolution in the material.