Abstract: The diffuser is one of the key components of the supersonic and hypersonic wind tunnels, which directly affects the efficiency-cost ratio of the wind tunnel operation. However, for the hypersonic low density wind tunnel, the range of operating parameters is large and the test gas density is relatively low, and thus the effects of the conventional diffuser with "contraction section, equi-straight section and expansion section" are not obvious. In this paper, a new central-type diffuser structure is presented, and the efficiency of the central-type diffuser is tested at the Φ300 mm hypersonic low density wind tunnel for M16 small flow rate states and M8 large flow rate states. At the same time, the influence of the test section model on the diffuser's capacity is analyzed. The results show that the central-type diffuser has a wide range of operation parameters and a significant pressure increase efficiency, and can effectively improve the wind tunnel test capability. This study can provide a reference for the design of the hypersonic wind tunnel diffuser.