Abstract: A 2D numerical model was established for the diffuser of a 1 100 MW nuclear power turbo-generator using CFD commercial software, so as to study the flow characteristics and performance parameters of the preliminarily designed diffuser under design conditions. Simultaneously, profiles of the both side planes were parameterized and analyzed according to the cubic Bezier curve, during which a group of profiles with excellent flow properties were obtained. Results show that for different types of profile, the main parameters play opposite roles in influencing the flow properties. Compared with the preliminary diffuser, the outlet velocity distribution of optimized diffuser has been improved greatly, with a reduction of total pressure loss coefficient by 83.92% and an increase of static pressure recover coefficient by 20.83%.