The invention discloses a self-holding type friction-stir welding stirring head with a lower shaft shoulder floating along with welding and a welding method, and relates to a friction-stir welding stirring head and a welding method. The friction-stir welding stirring head and the welding method aim at solving the problems that in the welding process, the distance between an upper shaft shoulder and a lower shaft shoulder in existing self-holding type friction-stir welding can not be adjusted and consequently the plate thickness is nonuniform, severe trimming defects occur, and thus the joint bearing capacity is lowered, or a stirring needle fractures in the welding process. According the stirring head, an upper shaft shoulder and a stirring needle are coaxially and fixedly installed together, the floating lower shaft shoulder is installed on the stirring needle in a sleeving mode, and the lower portion of the stirring needle is connected with a gas cavity; a first sealing ring is arranged in the matching position of the center of the floating lower shaft shoulder and the stirring needle, and a second sealing ring is arranged in the matching position of the floating lower shaft shoulder and the gas cavity. The welding method comprises the steps that 1, the stirring head is assembled; 2, friction-stir welding is conducted; and 3, the stirring head is disassembled. The friction-stir welding stirring head and the welding method are suitable for friction-stir welding.