Abstract: Laser isothermal quenching test was conducted on hot-rolled Q355B steel plate with a thickness of 40 mm. The microstructure and microhardness of Q355B quenched samples were observed using OM, XRD and Vickers hardness tester.The influence of laser quenching temperature and scanning speed on the surface quenching depth and hardness of Q355B steel was studied. The results show that the matrix structure of the rolled Q355B steel is composed of ferrite and cementite, and the surface hardening layer structure after laser quenching treatment is composed of martensite and cementite. The increase of laser quenching temperature and the decrease of scanning speed can both increase the depth of the quenching layer, and the microhardness shows a trend of first increasing and then decreasing. After treatment with the optimal quenching temperature of 1250 ℃ and scanning speed of 3 mm/s, the quenching depth of Q355B steel is 1127 μm. The hardness at a depth of 0.1 mm-0.2 mm from the quenched surface of the steel is 402 HV0.2-382 HV0.2, which is about 2.4 times of the substrate.