The utility model designs a steel tube clamping mechanism. When the mechanism is matched with a lathe for use, steel tubes can be subjected to welding groove processing so as to realize butt welding between the steel tubes. The steel tubes capable of being clamped are mainly used for conveying pipes for high temperature resistance, low temperature resistance and corrosion resistance in the industries of petroleum, chemical engineering, electric power and a boiler, is suitable for clamping and processing various grades of pipes made of carbon steel, stainless steel, alloy steel and the like and has wide clamping range. In the utility model, three groups of pawls are utilized to clamp the steel tubes; the diameters of the steel tubes are adjustable; the clamping force is adjustable and can be automatically compensated; and the steel tube clamping mechanism is firm to clamp and has good centering property; the clamping of the steel tubes is driven by a worm gear and worm speed reducer; the effect of self-locking after clamping can be realized; and therefore the phenomena of slipping of a workpiece and the like in the traditional clamping mechanism are prevented. The steel tube clamping mechanism has the characteristics of light weight, high efficiency, wide processing range and the like.