The types and amounts of chemical substances used are increasing day by day due to the sophistication of industry, and many harmful substances are derived not only from these chemical substances but also from pesticides and pharmaceuticals. In particular, pharmaceuticals are widely used not only by humans but also by domestic animals and pets, and there are various routes by which residual pharmaceutical substances after use are discharged into the water system. The purpose of this study is to present a method for assessing the danger of ecosystems using the Similar MOA formula when toxic substances are present in a mixed water system rather than individuals such as pharmaceuticals. The research results are as follows. The LOEC for the test species of the Ibuprofen and Carbamazepine mixture is 4.00 mg/L for algae (Pseudokirchneriella subcapitata), 2.88 mg/ L for Daphnia magna, and 1.57 mg/L for Danio rerio's embryo and E(L)C50 is 58.39 mg/L, 35.84 mg/L and 15.37 mg/L, respectively, and the PNEC based on this is 0.1537 mg/L. Ibuprofen and Carbamazepine are displayed at 0.0402 ug/L and 4.9081 ug/L in the Youngsan River system. The respective HQs are 0.00007 and 0.3. However, when these were mixed, the conversion value HQ was 0.6, and it increased significantly compared single substance. Various kinds of harmful substances are mixed and exist in the water system, but if we know the mixing ratio of each single substance with E(L)C50 then we can effectively perform ecological risk assessment of mixtures with through the use of MOA formulas in this paper.