The utility model relates to a battery box assembly of a passenger vehicle. The battery box assembly comprises a battery box, a tray and a locking mechanism of the battery box, wherein the locking mechanism comprises a lock plate which is fixedly arranged on the lateral surface of the battery box and a lock sheet which is arranged on the tray in a rotational mode; the upper surface of the lock plate is a curved surface of which the bulged part faces the top of the battery box; the lock sheet comprises a connecting plate which is arranged on the tray in a rotational mode and a compression joint part which is fixedly arranged at one end of the connecting plate; and the compression joint part has an internal surface of which the fit clearance with the curved surface is reduced gradually to press the curved surface downwards in the process of moving towards the peak of the bulged part of the curved surface from one side of the curved surface. Simultaneously, the utility model also provides the locking mechanism for the battery box of the passenger vehicle. The locking mechanism is convenient to operate and simple in structure, so that the problems of complex structure and inconvenience for use in the locking mechanism for the battery box in the prior art are solved.