Abstract: Based on the investigation of Branchiostoma balcheri resources in Xiamen Amphioxus Natural Reserve in August 2019(summer),November 2019(autumn),January 2020(winter) and April 2021(spring),this paper studied the resource status and the reasons for resource changes of B.balcheri,combined with historical survey data.The results showed that the average body length and weight of B.balcheri were 29.71 mm and 68.0 mg, respectively.The population was mainly composed of two years classes and three years classes, accounting for 76.2%.There were significant differences in the average body length and weight in the four quarters.The average density of B.balcheri in Eyu Island sea area was 50.0 ind/m²,which was the highest of the four sea areas in the reserve, but the range was very small.The average density of Huangcuo, Nanxian-shibaxian and Xiaodeng Island-Jiaoyu Island sea area were 4.1,1.5 and 0.4 ind/m²,respectively, which decreased significantly compared with the historical survey, especially Xiaodeng Island-Jiaoyu Island sea area did not appear basically.The change of habitat environment was the main reason for the decrease of B.balcheri resources in Xiamen sea area.