The invention discloses a machining process for a double-layer composite metallic tube. The process sequentially comprises the following steps that an outer metallic tube body and an inner metallic tube body are placed into an inner tube model cavity; high-pressure water is led into the inner metallic tube body so as to finish inner high-pressure forming of the inner metallic tube body, and a primary semi-finished double-layer composite metallic tube is prepared; the primary semi-finished double-layer composite metallic tube is placed into an outer tube model cavity; high-pressure water is led into the outer metallic tube body so as to finish inner high-pressure forming of the outer metallic tube body, and a secondary semi-finished double-layer composite metallic tube is prepared; and end post-treatment is conducted on the secondary semi-finished double-layer composite metallic tube, so that the finished double-layer composite metallic tube is prepared. Two passes of inner high-pressure forming are adopted, the forming effects of the inner metallic tube body and the outer metallic tube body are guaranteed, the machining procedures are optimized, and the machining process is more concise, so that the machining efficiency is improved, the profit rate is increased, and the product rejection rate is decreased.