Abstract: 08Cr9W3Co3VnbCuBN (G115) steel is the only martensitic heat-resistant steel in the world that can be engineered for efficient ultra supercritical units at 630 ℃. The high-temperature yield strength of G115 steel is higher than that of A335P92, making it difficult to form large-diameter hot extruded tees. We studied the material specification design method, prefabricated hole shape and size design method, radial extrusion forming process, and branch pipe shaping process of G115 large-diameter hot extrusion tee. Based on finite element analysis method, we simulated the forming process of G115 large-diameter hot extrusion tee. A G115 large-diameter hot extruded tee with specifications of ID902mm × 38mm/ID902mm × 38mm was trial produced. After normalizing and tempering heat treatment, the product's appearance, mechanical properties, hardness, metallographic examination, and non-destructive testing all met the design technical requirements.