Based on steel-concrete composite beam,mathematical model is set up according to plastic neutral axis location.Furthermore,multi-objective optimization method is used to carry out optimized design of steel-concrete composite beam section size in the cyanidation workshop of a gold mine in Liaoning.The construction cost of the optimized composite beam design is 8.0 % lower than ordinary calculation results,the weight is 5.8 % lower,and the design index is relatively good.The design provides reference for the application of steel-concrete composite beam in each kind of platforms and floors such as ore-dressing plant and workshop.