The invention discloses a preparation method of a metal tube, which is characterized in that the method comprises the following steps: (1) pouring metal liquid to a mould with a mold core, and preparing a metal tube blank; (2) removing the mold core and oxide scale; (3) under the condition of inert gas, heating to alloy plastic deformation temperature range, and keeping the temperature for 1 to 3 hours; (4) coating a layer of lubricant, and then performing hot extrusion; (5) cropping, straightening and removing the oxide scale by scouring to obtain the finished product metal tube. The method of the invention directly casts the metal tube blank with opening structure, improves the utilization ratio of material, simplifies the manufacturing process of the metal tube and saves production cost, and prepared metal tube has high appearance quality.