The effectiveness of the classic screening strategy was verified, by duplicating and verifying the degradation of gossypol by the Aspergillus niger. It can reduce the free gossypol content through biosorption but has no effect on the total gossypol content and cannot effectively degrade gossypol. And the most interesting thing we found the strain can secrete agarase, utilise agar as carbon source. In this case, that will mislead researchers and lead them to make wrong judgments. That turns out the usual methods of previous screening strategies are not rigorous enough, the classic screening method has defect in screening toxin-degrading strain, so agar control group should be added. In this study, some suggestions are put forward to optimise the same type of experiments and broaden the idea of detoxification by microorganisms and provide reference for screening effective toxin-degrading microorganisms.