The external gravitational potential of the earth is shown by the spherical harmonics model. In general，the parameters of the model are γ、θ、λand the polar is the North Pole. When the model is app1ied directly to the calculation of the free f1ight trajectory，since the recurrent calculation can not be avoided，an increase in orders of model wil1 result in a decrease in the speed of calculation. In this paper，the transitional method of the spherical harmonics[2] is applied and a new model is gained. The parameters of the new model are γ(geocentric radius)，β(range)，ζ(cross-deviation)and the polar is decided on after choice-making. With some approximations the new model is now being applied in the calculation of trajectory. With the recurrent calculation got rid of，the speed of calculation is increased. Practical calculations have shown that when the orders of the model are 30，the application of the new model enables us to enhance the speed of calculation 6 or 7 times and the error will be no more than several meters.