Because of the tiny trauma and rapid recovery, punctures have been widely adopted in clinics. The flexible needle is an improvement of the traditional steel needle. We firstly build the kinematic model of a flexible needle and use unscented Kalman filter (UKF) to estimate the states of the flexible needle tip. Then, a control method based on the reachable region of flexible needle is developed according to the characteristic of the kinematic model of the flexible needle. We analyze the reachable parameter's effect on the puncture trail. Simulations and gelatin puncture tests show that this control method is precise and robust. The tip error will be confined to 0.5 mm when the insert distance is 100 mm. Simulation results also indicate that the control method can achieve high-precision puncture when the error between the model parameter and actual value is within a certain range.