The species sensitivity distribution (SSD) method was used to identify the optimal positive references of the typical four aquatic biological toxicity tests, including luminescent bacteria toxicity test, algal growth inhibition test, daphnia immobilization test and fish mortality test, and their PNECs were estimated. The results showed that the best positive references for the four biological toxicity tests were zinc sulfate, copper chloride, potassium dichromate, and mercuric chloride, respectively. Their PNECs for the whole species are 114.0, 11.0, 16.0 and 7.30 μg/L, respectively. This research will provide reliable toxicity score standards for researchers who use group ecotoxicity for water quality safety assessment, which would make the assessment method more convenient and effective.