Abstract: The paper presents the computation flow of GA-PSO (Genetic Algorithms-Particles Swarm Optimization) hybrid algorithm and the fitness function determination for modal updating based on GA (Genetic Algorithms) and PSO (Particles Swarm Optimization) algorithms. A five-layer steel frame construction is used as an example and it is shown that this method enjoys a high efficiency. The GA-PSO algorithm uses the GA to efficiently search for the global-optimization solution at an early stage, and on the basis of the GA solution, the PSO algorithm is used to intensively search for the local-optimization solution at a later stage. Comparing with the PSO and GA, GA-PSO algorithm enjoys higher updating efficiency and precision.