After the internal parts of the Mafi-trench gas expander were badly damaged and became inoperative during the commissioning, the user entrusted Sichuan Air Separation Group for local repairing of the expander, since the foreign company set forth rigorous requirements for repairing.The present study focuses on the in-depth analysis and comparison of the part structures of the unit and the 3-D flow passages of the impeller on the basis of the expander design concept of Sichuan Air Separation Group and finally offers the modification design.The modified expander works well on the site and creates significant economic benefits for the user,which proves that the idea of modification design of Sichuan Air Separation Group is feasible and provides a new idea and method for local repairing of exported LPG expanders.