Abstract: To quantitatively detect the cytochrome b (Cyt b) gene of Prorocentrum donghaiense Lu in the filed samples, a specific primer was designed, and the quantities of the RNA templates added into the reaction system for reverse transcription as well as the reaction conditions of realtime fluorescent quantitative PCR (RFQ-PCR) were optimized. The results illustrated that the designed primer had good specificity, being able to be used to differentiate different algal species effectively. In detecting the filed samples, the suitable qualities of the templates for the 20 μL reverse transcription system were 50-200 ng. 10fold diluting the templates or adding the bovine serum albumin (BSA) with a final concentration 0.2 μg·μL^-1 into the RFQ-PCR system could effectively decrease the inhibitory effect of the inhibitors in the filed samples, and thus, decrease the interferences. The established realtime fluorescent quantitative PCR (RFQ-PCR) assay would facilitate us to study the intrinsic mechanisms of P. donghaiense outbreak and extinction at molecular level.