A bicycle transmission device for connecting to a freehub body on a rear wheel, which has convex keys arranged radially on the periphery. The bicycle transmission device includes a connecting member, a fastener, and a cogset assembly. The connecting member includes a cylindrical body portion, connection keys protruding from a periphery of the body portion, a cut groove passing through the body portion, and first and second connecting seats located on two sides of the cut groove. The inner wall of the body portion has concave grooves corresponding to the convex keys. The fastener is adapted to be connected to the connecting seats and to pull them together. The cogset assembly includes a cogset seat and sprockets. The cogset seat has a first socket hole fitting around the connecting member and connection grooves corresponding to and adapted to be connected to the connection keys. The sprockets with different numbers of chain teeth are adapted to be connected to the cogset seat.