A cutting knife assembly includes a motor, a knife, a fixing base and a position adjustment stand. The motor has a converter, the knife has a blade portion and a body portion connected with the blade portion and the converter. The fixing base has a main body having a penetration channel and a screw structure. The knife and the converter are penetrated through the penetration channel, the blade portion is exposed outside the main body, and the screw structure is disposed around the penetration channel. The position adjustment stand has a through hole having an inner screw structure matched with the screw structure. The main body and the through hole are screwed with each other. When the converter is driven by the motor, the knife and the fixing base are rotated by the converter, thereby adjusting an exposed distance. The height of the knife can be precisely adjusted.