The invention provides a gear train tensioning device and a gear train. The gear train tensioning device comprises: a first rotating member for driving connection with a second rotating member via a belt so that a gear train or a part of the gear train is formed by the first rotating member, the belt and the second rotating member; and an elastic member disposed between a bracket and the first rotating member, the elastic member urging the first rotating member to move the first rotating member away from the second rotating member in a direction of tensioning the belt; the elastic member includes a spring, and the spring force direction of the spring is substantially parallel to the connecting direction of the rotation center of the first rotating member and the rotation center of the second rotating member. The first rotating member can be forced by a small elastic member, and the first rotating member can be easily moved away from the second transmission member in the direction of belt tension by a small force, so that the belt tension is obvious, and the structure of the gear train tension device is simplified, the weight is reduced, and the cost is reduced.