A calculation model is presented for predicting fuselage lateral mode frequency of helicopter during running on runway. The model is verified from shake test data at stayed condition, and the maximum theoretical error is less than 10%. Trim analysis shows that the helicopter steady running can be built up with only a small collective pitch pilot. Analytical results demonstrate that the decrement of lateral stiffness of tires is a primarily factor that leads the second lateral mode frequency decresing with the increase of running velocity. Especially in the low velocity margin, the second lateral mode frequency quickly drops as velocity increasing. Compared with the stayed condition, the mode frequency has 47.6% decrement at running velocity of 35 km/h.