A wavelet transformation-based fuzzy reflex control method is proposed to prevent slip and maintain stable grasping posture for prosthetic hands. A key component of the control method is that the response time from slip detection to slip prevention is very short. The proposed control method mainly includes slip detection, reflex grasping force estimation, and use of a reflex controller. In the slip detection, initial slip information can be rapidly obtained via wavelet transformation as soon as disturbances cause the grasped object to slip. This shortens the response time of the reflex control. The initial slip information can then trigger the reflex control. In the grasping estimation, a reflex force estimation model is presented to estimate a force increment that is used to adjust to the desired force. In the reflex controller, a fuzzy logic controller is adopted to track the desired force. Finally, the proposed control method is applied to control a prosthetic hand with a single degree of freedom. The experimental results show that the proposed control method can adjust the grasping force and effectively prevent slip under disturbances. The time of slip prevention is less than 160 ms.