In order to accurately calculate the result of gust load, a mathematical model of nonlinear system for helicopter flight dynamics is developed with the consideration of both temporal derivative and spatial gradient of variable wind velocity. The gust load for helicopter is analyzed through the time-domain overload coefficients simulation, which is compared with the result from engineering estimation formula. By consideing the influences of helicopter rotor, tail rotor, fu selage, vertical stabilizer and horizontal stabilizer on gust load, this method provides more accurate result of gust load in a forward flight with small advanced ratios. The comparison result shows that the vertical gust load has more significant influences on helicopter structural strength than gust in other directions. Moreover, the relationship between gust load and gust velocity strength is linear under the same advanced ratio. Numerical results indicate that the proposed method can be applied to analyze gust load in different flight states and variable gust fields.