The invention discloses an S-curve acceleration and deceleration planning method for coping with a negative value of the start/ end speed. The method solves the problem that in the prior art, when thestart or end speed in machining is negative, the speed and time can only be planned in a subsection manner, so that machining efficiency is influenced. The method can carry out motion planning easilyand quickly under the condition that the start or end speed is negative, and is not affected by acceleration constraints when the speed is close to zero, and thus machining efficiency is improved. The technical scheme is characterized by, when the at least one of the start speed and the end speed is negative, serving the smaller one of the start speed and the end speed as time shaft downward movement distance; and then, calculating displacement S1 under the current start and end speed according to S-curve acceleration and deceleration, and obtaining planned displacement S according to the S1and translational region area.