A bicycle trainer includes a base frame, a damper unit pivotally connected to the base frame and including a rotating wheel set, a swivel frame pivoted to the base frame and biasable between an inclined position where the swivel frame and the damper unit are capable of supporting a bicycle wheel and a collapsed position where the swivel frame is received in the base frame and the rotating wheel set of the damper unit is disposed adjacent to the swivel frame, and a locking device for locking the damper unit to the base frame or allowing the damper unit to be biased relative to the base frame. Thus, the composition of the invention allows the bicycle trainer to be folded up and received in a compact condition to minimize the dimension convenient for carrying with less effort.