On MTF thoery of the pushbroom imaging system using the CCD line array, the formula which is used to calculate, respectively, the MTF in the line direction and in the direction perpendicular to the line direction has been given in some articles, under a special condition of pushbroom motion perpendicular to the line. In this paper author derives a general formula which is valid for any direction and any angle crossed by the directions of the line array and pushbroom motion.