The invention provides a cavity wall evaporating pipe of a combustion chamber of a mini-sized engine. By means of the cavity wall evaporating pipe, the contact area of gas flow and the evaporating pipe can be effectively enlarged, heat exchange is enhanced, the mixed gas standing time is prolonged, and the fuel oil atomization and evaporation effects are improved. A regular cavity structure is designed for the inner wall of the evaporating pipe, the area of the inner wall is enlarged, and space is increased. Fuel mixed gas flow forms backflow areas in cavities after flowing through the evaporating pipe, the speed is decreased, the mixing space of fuel oil and air is increased, the standing time in the evaporating pipe is prolonged, and the atomization and evaporation time is prolonged; meanwhile, the contact area is enlarged, and the heat exchange effect is enhanced. Bosses are designed on the portions, at the same positions of the cavities, on the outer wall of the evaporating pipe, the strength of the evaporating pipe is ensured, the area of the outer wall can be enlarged, heat exchange between high-temperature gas flow in a flame tube and the evaporating pipe is enhanced, the wall temperature of the evaporating pipe is raised, and the fuel atomization and evaporation effect in the evaporating pipe of the combustion chamber of the mini-sized engine is enhanced through the effects of the two aspects.