Abstract: High-strength Al-Cu alloys are often used in the fuel tanks of heavy-lift launch vehicles, and the friction stir welding (FSW) process is used to achieve the welding. With the increasing size of the tanks, the difficulty of welding Al-Cu alloys by adopting the thickness compensation process increases dramatically, and the lightweight advantage of large-thickness aluminum alloy tanks is not significant. Stainless steel materials, by virtue of the mature industrialization technology and low difficulty of the welding process, exhibit certain advantages in the field of large tank manufacturing. 3 mm-thick 316L austenitic stainless steel material was used as the test object, and the dynamic recrystallization mechanism and mechanical properties of the welded joint microstructure under different welding speeds were studied. The results show that the complete dynamic recrystallization occurs within the FSW joints at different welding speeds; with the increase in welding speed, the discontinuous dynamic recrystallization mechanism in the weld core region is significantly inhibited, while the continuous dynamic recrystallization mechanism is enhanced. The FSW process can be used to obtain equal strength joints with strength reaching more than 98% of that of the base material; the strength of the weld core region is greatly improved, and the joints are insensitive to weld defects, which confirms the feasibility and performance advantages of austenitic stainless steel material for large rocket tanks under the FSW process conditions.