Abstract: A new algorithm-optimal control algorithm of terrain following（TF）system was presented by Funk, J. E. and Kelly, R. T. in 1975.This algorithm is used to design the desired flight path（DFP）with cubic spline from the sampled data of terrain contour, then the aircraft is forced to fly along this path by corresponding optimal controller. Since then, increasing attention has been paid to it, furthermore, it has been applied to terrain following/terrain avoidance system.Because this algorthm uses cubic spline to design DFP, it is difficult to consider constraint conditions, such as the normal acceleration limit & nd the flight trace angle limit of vehicle. Consequently, it needs long computation time, then can not satisfy the requirement of real tim e calculation.This paper presents a method of designing DFP with circular arc spline. Under forty terrain conditions, This method is compared with the DFP of cubic spline and proven to be feasible.This new method decreases needed computation time, thus making real time calculation possible.