800MPa grade RPC steel is the recently developed new generation steel, which was produced by relaxation-precipitation controlling transformation (RPC) processing. The microstructure and mechanical properties of the heat affected zone (HAZ) in laser welding of 800MPa grade RPC steel were investigated by using a 4kW YAG laser and a Gleeble-1500 thermal simulator in this paper. The experimental results indicate that: (1) The size of prior austenite grains of HAZ increases with increasing heat input. (2) Granular bainite is the main microstructure of the HAZ of laser welding. (3) As the heat input increases, the hardness of HAZ decreases, but it is higher than that of the base metal, indicating no softened zone after laser welding. (4) As t8/5 increases, the impact toughness of HAZ increases at first and then decreases. The impact energy of HAZ is much higher than that of the base metal when t8/5 is between 3s and 8s. It indicates that excellent low temperature toughness can be obtained under appropriate laser welding conditions.