Abstract: Malachite green (MG) is a synthetic compound with a green metallic sheen, widely applied to enhance production and protect aquatic animals from various diseases. However, MG and its primary metabolite, leuco-malachite green, are toxic inorganic contaminants hazardous to human and other organisms' health. Residual MG in aquaculture water has been identified as a significant contributor to severe water pollution. Since 1981, MG has been designated as a Class II Health Hazard due to its teratogenic, carcinogenic, and mutagenic impacts. Currently, numerous countries, including the European Union, the United States, and China, have banned the use of MG in aquaculture practices. Nevertheless, its low cost and high effectiveness have led many culturists, driven by economic interests, to continue its widespread use as a medication in fish farming, particularly in some developing countries. In recent years, China has experienced outbreaks of MG residues exceeding acceptable levels in aquaculture environments, prompting increased attention to the quality and safety of aquatic products. In 2002, the Ministry of Agriculture of China listed MG among prohibited veterinary drugs. In this work, a fluorescence resonance energy transfer system using poly(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo2,1,3thiadiazole-4,8-diyl) (PFBT) polymer dots as the energy donor and MG as the acceptor was established to observe the fluorescence quenching phenomenon of PFBT polymer dots. The PFBT-MG system maintained good stability in Britton-Robinson (BR) buffer solution at pH 5.0 to 9.0. Under optimized conditions, the system's fluorescence showed a strong linear correlation with MG concentrations ranging from 0.04 to 0.59 mg/L. The detection limit was as low as 1.38×10-4 mg/L, comparable to other methods. The correlation coefficient (R2) was 0.992 in our method. Simultaneously, common ions such as K⁺, Ca²⁺, Na⁺, Mg²⁺, Cu²⁺, Fe³⁺, and Cl⁻ in water showed minimal interference with the fluorescence intensity of MG detected by PFBT. The method has been successfully applied to determine MG in aquaculture freshwater, seawater, and transportation water, achieving a recovery rate of 96.6% to 103.1%. The relative standard deviation was less than 1.05%. Therefore, it could be concluded that PFBT polymer dots are a reliable and convenient method for rapid detection of MG. This method provides a more accurate fluorescence technique for detecting MG. The synthesis of PFBT polymer dots does not require abundant reagents for elution and avoids the costs of expensive equipment. The entire process only requires imaging equipment, making it suitable for large-scale promotion economically. At the same time, PFBT polymer dots significantly enhance detection efficiency, with the detection process not exceeding 30 minutes for imaging and observation, thus enabling rapid on-site detection. Most importantly, PFBT polymer dots exhibit good sensitivity, selectivity, and anti-interference capabilities in the MG detection process, providing an important reference for the future inclusion of on-site MG detection.