According to the present High Frequency (HF) Air-Ground Communications, this paper derives the function relation between aircraft flying position and time, simultaneously proposes a new frequency changing strategy based on SNR sorting. This new strategy based on ITS area SNR data ascertains the distribution curve of each channel's SNR along time on the route through interpolation method. At the same time, real time frequency application plan on the whole route of plane is given by this strategy, thus ensuring that the channel communication quality should be maintained at a high level, and the REL be regarded as the evaluation standard of channel quality. Simulation results demonstrate the great improvement on average SNR and communication reliability on the route, which insures communications during flying always being at the optimal channel and overcomes the disadvantage that one frequency only fits one area's communications.