The utility model relates to a device for protecting passengers on a vehicle. An actuator (32) of an active knee pad (24) comprises a drive power generating device (38) and a one way lock mechanism (40) arranged nearby the working end of the drive power generating device (38), wherein the one-way locking mechanism (40) is formed by the way that if a piston member (42) is moved in an inward insertion direction, every splicing part (64) is moved to a bevel part (62B) by the thrust force of a spring (66), and accordingly, the splicing parts (64) are abutted against the peripheral surface of the piston member (42), so that the piston member (42) and the splicing parts (64) are fixed. Accordingly, since the one-way locking mechanism (40) and a main body part (30) of the active knee pad (24) moved to a restraining position can not return along a vehicle forward direction, and thus, large force for restraining passengers' lower limbs (28A) can be generated.