Abstract: The dynamic characteristics of the transition process and the cost of surge chamber construction are the two main aspects to be considered in the design of the surge chamber body. Selecting the parameters of the surge chamber structure to optimize both of these aspects is of great significance for the construction and operation of hydropower stations. To address this challenge, a surge chamber body shape optimization strategy based on the Non-dominated Sorting Genetic Algorithm III（NSGA-III） and Technique for Order of Preference by Similarity to Ideal Solution（TOPSIS） is proposed.This strategy considers the optimality of both the dynamic characteristics of the transition process and the engineering investment. Firstly, a refined model of the hydroelectric system is established based on the feature line method. Secondly, by taking the surge chamber diameter, impedance hole diameter, and installation position as decision variables, the paper establishes a large fluctuation optimization model targeting large fluctuation dynamic characteristics（Maximum head of unit+Maximum rate of increase of unit speed） and surge chamber volume, as well as a small load disturbance optimization model targeting rotational speed overshoot and ITAE. NSGA-III is introduced to obtain the Pareto solution sets under the two optimization models. Finally, using the TOPSIS evaluation method, the Pareto solution sets are further evaluated with the above four optimization objectives as the index system. This process leads to the identification of the surge chamber body type that considers the optimality of both the dynamic characteristics of the transition process and economic factors. The results indicate that, compared with the lowest-scoring scheme, the optimal scheme can effectively improve the integrated regulation characteristics of large and small fluctuations under the premise of sacrificing a small surge chamber economy, and realizes the goal of balancing the construction cost of surge chamber and the integrated regulation characteristics of large and small fluctuations.These findings are of great significance in guiding the structural design of surge chambers in hydropower stations.