The invention discloses an opening-closing device for a bottom door of a vehicle and belongs to vehicular door mounting and demounting techniques. The device comprises a vehicular bottom door opening-closing mechanism and a bottom face facility structure. The vehicular bottom door opening-closing mechanism comprises a pair of duplex screws, a door opening arm, a door closing arm, a spindle, a long connecting rod, a short connecting rod, two elastic devices and two bottom door sheets. The bottom face facility structure comprises a door opening contact plate and a door closing contact plate which are positioned on two sides of the vehicle respectively, the door opening arm contacts with the door opening contact plate, and the door closing arm contacts with the door closing contact plate. Self-locking of the bottom door is completed under intrinsic locking force of the vehicular bottom door opening-closing mechanism, automatic unloading during low-speed running of the vehicle is realized, and unloading is quick while unloading efficiency is high.