In order to solve the problem that lowtemperature impact toughness of traditional highstrength lowalloy Hbeam is lower, from taking account of composition design, boron was added into this type of steel for the first time, and the effect of boron on microstructure and mechanical properties was investigated. The results show that compared with nonboron steel the strength and the plasticity of boroncontained steel are increased slightly, moreover the impact toughness is improved greatly, especially at low temperature. The microstructure is refined to a certain extent because of the smaller and evenly distributed Nb（C,N）precipitates and also the brittle fracture of the materials is restrained by boron addition, so that the ductilebrittle transition temperature is significantly reduced.