Abstract: In the research of sewage treatment submersible mixer at domestic, hydraulic thrust, torque, rotational speed and other parameters are common parameters whereas the calculation formula and related concepts for the efficiency have not been given. However, the efficiency is a necessary parameter for every machine. Based on the analysis of the submersible mixer power losses, the efficiency formula of submersible mixer was deduced through the power definition and conservation of momentum. Relationships that the efficiency was proportional to 3/2 power of the water thrust and inversely proportional to the impeller diameter and shaft power of the submersible mixer were obtained. Meanwhile, the efficiency of 25 kinds of submersible mixers widely used at domestic was analyzed through this formula. It was found that the efficiency of these mixers was generally low and four-fifths was less than 60%. The submersible mixer efficiency was simulated numerically, and the result suggested that as the average velocity of the pool fluid was 0.291 m/s, the efficiency reached 62.43%. Definition and discussion for the formula of the efficiency of sewage treatment submersible mixer provides a reference for the design and application of submersible mixers in the industry.