The invention discloses a combustion jet cutter used for steel structure member cutting. The combustion jet cutter consists of a jet cutter bottom, primer, a shell case, ignition agent, cutting agent and a nozzle, wherein thermite is based on plain thermit in the cutting agent; the plain thermit roughly accounts for 65-85wt% of the cutting agent; gas-forming agent is based on potassium nitrate which roughly accounts for 5-25wt% of the cutting agent; and slagging constituent and alloying agent roughly account for 7-20wt% of the cutting agent. During cutting, the combustion jet cutter is filled into a cutting gun, the cutting gun is held with a hand to aim at a cutting part, the jet cutter primer is triggered, inner ignition agent the jet cutter is ignited by the primer, the cutting agent is ignited by the ignition agent to generate high-temperature metallurgical melt and high-pressure gas, and a product is injected after being subjected to energy-gathering compression by the nozzle to locally melt the steel structure member so as to realize cutting. The jet cutter is mainly used for the quick thermal cutting and the underwater cutting of various steel structure members.