Alexandrium catenella is an important toxic algal species in the Alexandrium genus, which can form toxic red tides in large numbers. The paralytic shellfish poisoning (PSP) produced by Alexandrium catenella can seriously endanger human health and threaten the production and development of the aquaculture and fishery industries. Therefore, it is important to explore and develop effective detection and early warning methods for toxic red tides. Traditional microscopy and molecular biology detection methods require specialized technicians and equipment, which are time-consuming and complicated to operate and cannot be adapted to the needs of rapid on-site detection. This study utilized the colloidal gold immunochromatography technique to prepare polyclonal and monoclonal antibodies against Alexandrium catenella, which were then assembled into colloidal gold test strips. These test strips enable rapid detection of Alexandrium catenella in aquatic environments and facilitate semi-quantitative analysis of its concentration. They allow for a quick assessment of whether the algal concentration in the environment has reached the baseline for red tide conditions, facilitating the timely implementation of preventive and control measures. This method holds significant practical importance and promising application prospects.