Abstract: For shortterm forecast of available frequency in shortwave communication, a shortterm ionospheric forecast method based on Kalman filter assimilation technique has been proposed. According to the diurnal variations of ionospheric parameters sounded by ionospheric sounding network, shortterm forecasting of critical frequencies of foF2 in the next 1~24 hours has been accomplished successfully. The shortterm forecasting results of China region during three epochs of the solar activity are analyzed， and the results show that the rootmeansquare error of forecasting value and observation of ten sounding stations was from 0.57 MHz to 0.72 MHz; the relative error was from 9% to 12%, and the forecasting accuracy of region of medium and high latitude was better than that of low latitude.