Envelop grasping model based on 3-DOF fingers driven by a flexible pneumatic actuatordirectly was proposed. Based on the principle of equalization, the interaction forces between the object and knuckle was optimized. The static model of the pressured air in FPAs and the positive pressure on knuckle or correlative friction force was built. The measure model about the positive pressure and its position, based on leverage principle depending on two touch-force sensors, was obtained. Using a series dual loops control method and the compensator about friction force, the positive pressure could be controlled precisely. In order to filtrate the high-frequency noise, the feedback signal of the pressured air in FPA was processed by different analysis. The experimental results showed that dynamic response time of the positive pressure was about 1s and the steady-state deviation was less than ±0.5N.