The performance of reciprocating seals in rapping device of gasifier was studied through finite element method. The contact stress, Von-Mises stress, and friction power loss of O-ring and Sliding-ring combined Seal-ring were obtained. Meanwhile, the experimental study in performance of seal structures of rapping device were carried out. The results show that both of the seal structures are satisfied seal requirement, but compared with the Sliding-ring combined Seal-ring, the O-ring is easier to be destroyed and power loss of O-ring is higher than Sliding-ring combined Seal-ring. The results also verify the superiority of Sliding-ring combined Seal-ring in rapping device of gasifier.