The severe situations of the military struggle around the south sea of China are introduced at first. The necessities are discussed that HF communications should be used as a strategic communication. Then the distribution of the parameters such as radiation angle and maximum usable frequency (MUF) in the communication link between Yongshu island and other cities are analyzed in the conditions of four seasons. The change rules of MUF in day and night can be got from Haikou, Chongqing and Beijing. So the antenna forms and lowest usable frequencies can be decided for long distance and short distance link. At last the area coverage of signal to noise ratio (SNR) over Yongshu island is simulated.