We take earthquake energy activity field in northern China areas as a random function of space and time according to random function theory.The time weight coefficients curves and its space isopleths map of major typical energy field in the seismic activity region are studied with the expanding method of the natural orthogonal function.The results show that the variation of ascending or descending of the target curve may indicate the duration when the mid-strong earthquake occurs

while the variation of target isopleths density may reveal the area where the mid-strong earthquake occurs.The above-mentioned phenomenon will appear before most of the mid-strong earthquakes in northern China areas

and it confirms that the natural orthogonal function of the random earthquake energy field is one of the valuable methods to explore the earthquake forecast.