The utility model relates to a steam generator, in particular to a vehicle-mounted once-through steam generator with ultrahigh thermal efficiency, which is used for thermal recovery of underground thick oil in the petroleum industry. The vehicle-mounted once-through steam generator comprises a flue gas system and a water path system, an evaporation section, a convection section and a chimney in the flue gas system are sequentially communicated in the hot flue gas movement direction, the evaporation section is located above the rear portion of the hearth, and the convection section is located above the evaporation section. A convection tube bundle is arranged in the convection section, an evaporation tube bundle is arranged in the evaporation section, the convection section is divided intoan upper convection section and a lower convection section, the upper convection section is provided with the upper convection tube bundle, and the lower convection section is internally provided withthe lower convection tube bundle. The fuel consumption is reduced, the smoke exhaust temperature is reduced, the heat efficiency is improved, and the oil extraction cost is reduced.