The invention relates to the field of the boiler design and manufacture, and discloses a vehicle-mounted high-pressure DC steam injection boiler system with three-pass flue gas, which is applicable to the field of thermal exploitation of heavy oil in the oil industry. The boiler system comprises a trailer, a sleeve type water-water heat exchanger, a convection section, a high-pressure water supply pump, a combustion fan, a burner, a radiation section, a steam-water separator, an air preheater and a chimney, wherein the high-pressure water supply pump, the combustion fan, the burner, the radiation section, the steam-water separator, the air preheater and the chimney are arranged on the trailer in sequence along the length direction of the trailer, the cross section of the radiation sectionis square, the sleeve type water-water heat exchanger is arranged outside the lower side face of the front half section of the radiation section, the convection section comprises a left rectangular flue gas channel, a right rectangular flue gas channel, a lower rectangular flue gas channel and convection tube bundles respectively arranged in the three rectangular flue gas channels, and the left rectangular flue gas channel, the right rectangular flue gas channel and the lower rectangular flue gas channel are correspondingly arranged outside the left side face, the right side face and the lower side face of the rear half section of the radiation section.