To understand the low-temperature fracture behavior of the laser-MAG(metal active gas) hybrid welding joint of weather-resistant steel for high-speed trains, the integral values Jm of fracture toughness of the welding metal, heat-affected zone, and base material are obtained through a low-temperature fracture toughness test. The relationship between fracture toughness and temperature is fitted by the Boltzmann function, and the ductile-to-brittle transition temperature of each zone is obtained. The results demonstrate that the variation trend of the fracture toughness of each zone is reduced as temperature decreases. The base material has higher low-temperature toughness compared with the welding metal. The ductile-to-brittle transition temperatures of the welding metal and heat-affected zone are -65.9 ℃ and -70.4 ℃ respectively, both of which are higher than that of the base material (-81.9 ℃). The micro-fracture mechanism of each zone of the laser-MAG hybrid welding joint is explained based on the observation of the microstructure and fracture morphology. The poor low-temperature fracture toughness of the welding metal is primarily due to the large-size coarse grain and proeutectoid ferrite in the microstructure.