To analyze and optimize a large-scale wind turbine accurately，this paper considers the flexibility of the main shaft and the full flexibility of the gear transmission system，and establishes a flexible multi-body dynamic model of the drive chain of an 8MW wind turbine. The effects of number of condense points on the gear system modal are also studied. The natural frequencies of the system are obtained，and 6 dangerous resonance points are identified by applying five screening principles. Based on the characteristics of the resonance points，the gearbox model is optimized by comparing two methods. The number of resonance points is reduced to zero by optimization. The results can provide a reference for the dynamic modeling and optimal design of large-scale wind turbines