The Southwestern Atlantic Ocean （FAO Fishing Area 41） is rich in fisheries resources and serves as a major fishing ground for coastal and distant-water fishing countries and regions such as Argentina and European nations. Affected by global climate change， fishing activities， as well as the inherent life history characteristics of different fish stocks， the catch in this sea area has exhibited substantial interannual fluctuations over the past two decades. Investigating the catch composition structure and its interannual variability is of great significance for ensuring the sustainable utilization and scientific conservation of fisheries resources in the region. Based on the 2000-2023 fishery production statistics provided by the Food and Agriculture Organization of the United Nations （FAO）， this study analyzed the catch composition and interannual changes in the Southwestern Atlantic Ocean using the grey relational analysis method.The results showed that the grey relational grades of cephalopods， mollusks （excluding cephalopods）， pelagic fishes， demersal fishes and crustaceans were 0.911 0， 0.711 2， 0.627 3， 0.618 9 and 0.590 9， respectively. The species with the highest grey relational grade in each major group were as follows： Illex argentinus（0.952 6）， Zygochlamys patagonica （0.979 2）， Salilota australis （0.802 3）， Engraulis anchoita （0.797 3）， and Pleoticus muelleri （0.984 3）. It is suggested that cephalopods are the dominant factor affecting the variability of total catch in the Southwest Atlantic Ocean， while Illex argentinus， Zygochlamys patagonica， Salilota australis， Engraulis anchoita and Pleoticus muelleri are the key species driving the interannual catch variations of cephalopods， mollusks （excluding cephalopods）， pelagic fishes， demersal fishes and crustaceans， respectively. The results provide a scientific reference for the scientific management of fisheries resources in the Southwest Atlantic Ocean.