To study the influence of muscle activation on human neck injury under a specific braking condition, a mixed finite element dummy model was developed with detailed neck geometry and active and passive response capabilities of muscles. The kinematics and biomechanical responses of the occupant’s head and neck at relaxing and pre-tension states with a specific helmet under a specific braking condition were studied, respectively. The results indicated that pre-tensioning of the neck muscles before the start of braking reduced the peak values of the three kinematic parameters of head centroid horizontal acceleration, head relative to T1 thoracic spine horizontal displacement, and rotation angle by more than 20%, resulting in better stability of the neck during the entire moving process. From a biomechanical point of view, pre-tensioning the neck muscles reduced the maximum Von-Mises Stress of the vertebrae and intervertebral discs by 36.95% and 48.20%, respectively; the Nij value was reduced by 25.06%, which reduced the risk of neck injury in this braking condition.