A safe utility knife is provided having a housing, a blade sliding seat, a pushing force generator, an acceleration detector, a stopper, a stopper pushing portion, and a sliding switch. When activated with a blade protruding out, and an acceleration due to a sudden dropping movement occurs, the acceleration detector moves from a low acceleration position to a high acceleration position. It also presses down the sliding switch and the stopper so that the stopper moves from a locking position to a non-locking position. The blade sliding seat is pulled by the pulling force generator to a blade storing position to protect against accidental injury. The knife can be operated by one hand and is suitable for regular blades available on the market. In addition, it can be switched to manual automatic blade returning mode.