The invention relates to a vehicle-mounted ultra-supercritical pressure boiler. A combustion chamber is horizontally arranged, and an economizer is connected to the rear end of the combustion chamber through a variable-diameter connection flue; a combustor, the combustion chamber, the economizer and a chimney are communicated successively along a hot smoke moving direction; combustion chamber pipe bundles are arranged on the periphery in the combustion chamber; and the structure of a water-water heat exchanger is a casing-tube type S-shaped reciprocating pipe which is arranged above the outside of the combustion chamber for heat exchange. Feeding water is fed into a feed pump through a boiler water inlet, pressurized by the feed pump to 30.48MPa, flows through a water pipeline, the water-water heat exchanger, an economizer pipe bundle, the water-water heat exchanger and the combustion chamber pipe bundles to be heated gradually and reaches a saturated water outlet, and ultra-supercritical pressure saturated water at the temperature of 370 DEG C under the pressure of 30MPa is output. The vehicle-mounted ultra-supercritical pressure boiler can cover injection steam huff and puff of oil wells with an oil reservoir depth of more than 2200 meters and can operate under subcritical pressure to generate saturated steam with the steam dryness not higher than 80%, and accordingly, requirements for thickened oil recovery of ultra-deep oil wells are met.