The invention relates to protective device for an articulated vehicle (1) comprising at least a first and second vehicle portion (4, 6); a control gear (8) connected to the respective vehicle portion (4, 6); and an actuating member (16) for, via the control gear (8), changing the relative position of the vehicle portions (4, 6) in relation to one another. The protective device (2) comprises at least one stiffening means (22), which fixes the position of each respective vehicle portion (4, 6) in relation to one another whenever the acceleration and/or speed of at least one of the vehicle portions (4, 6) exceeds a predetermined limit value. The invention also relates to a method for controlling a stiffening means of such a protective device (2) for an articulated vehicle (1).