With a small molten salt lead-based reactor as its research object, this paper proposes a method combining Monte Carlo techniques and advanced optimization algorithms to realize the optimal design of the core. By selecting different core parameters and applying a multi-objective genetic algorithm, a more compact core design is achieved. Calculation results show that the proposed method can optimize the structure of a small molten salt lead-based reactor, realize the core optimization design under multiobjective constraints. The developed method can provide a favourable reference for the development and design of a new type of reactor.