The invention discloses a bottom door opening/closing mechanism, which includes a seat plate, a closing arm and a bottom door opening degree limiting device arranged on the seat plate, as well as a secondary locking device preventing the closing arm from rotating to the opening direction of the bottom door when the bottom door is in the closing state, wherein the secondary locking device is arranged on the seat plate, and can leave from the limiting position for limiting the closing arm. The bottom door opening/closing mechanism provided by the invention has the advantages that the secondary locking device arranged on the seat plate can be used for preventing the closing arm from rotating continuously when the eccentricity of a lever of a wagon with a bottom door is reduced to a set safety threshold value and the closing arm rotates to and is contacted with the secondary locking device under the action of the gravity of goods, so that the eccentricity value of the lever is not reduced any more, and further the situation that the bottom door cannot be opened automatically is guaranteed. The invention further discloses a bottom door wagon adopting the bottom door opening/closing mechanism.