A fitness bike with a braking device comprises: a frame; a flywheel rotatably mounted on the frame; an adjusting assembly mounted on the frame, and having a main housing mounted on the frame; a shaft movably mounted in the main housing; a button mounted on the shaft and located outside the main housing; and a first resilient member located in the main housing, receiving the shaft, and abutting the shaft; a braking assembly rotatably mounted on the frame, located next to the flywheel, and having an abutting element located next to the flywheel, and being capable of abutting a radial outer surface of the flywheel; and a wire mounted on the frame and connecting the braking assembly to the adjusting assembly, wherein one of two ends of the wire is mounted on the shaft, and the other end of the wire is mounted on the braking assembly: the fitness bike with a braking device characterized in that: the adjusting assembly further comprising a fine-tuning member located in the main housing, connected to the wire, and having an inner thread formed in the fine-tuning member; wherein the shaft is provided with an outer thread formed on an outer surface of the shaft and engaged with the inner thread of the fine-tuning member; wherein when the button is rotated, the shaft is rotated and thereby the fine-tuning member moves along the outer thread of the shaft thereby pulling or releasing the wire gradually. a frame; a flywheel rotatably mounted on the frame; an adjusting assembly mounted on the frame, and having a main housing mounted on the frame; a shaft movably mounted in the main housing; a button mounted on the shaft and located outside the main housing; and a first resilient member located in the main housing, receiving the shaft, and abutting the shaft; a main housing mounted on the frame; a shaft movably mounted in the main housing; a button mounted on the shaft and located outside the main housing; and a first resilient member located in the main housing, receiving the shaft, and abutting the shaft; a braking assembly rotatably mounted on the frame, located next to the flywheel, and having an abutting element located next to the flywheel, and being capable of abutting a radial outer surface of the flywheel; and a wire mounted on the frame and connecting the braking assembly to the adjusting assembly, wherein one of two ends of the wire is mounted on the shaft, and the other end of the wire is mounted on the braking assembly: the fitness bike with a braking device characterized in that: the adjusting assembly further comprising a fine-tuning member located in the main housing, connected to the wire, and having an inner thread formed in the fine-tuning member; wherein the shaft is provided with an outer thread formed on an outer surface of the shaft and engaged with the inner thread of the fine-tuning member; wherein when the button is rotated, the shaft is rotated and thereby the fine-tuning member moves along the outer thread of the shaft thereby pulling or releasing the wire gradually. the adjusting assembly further comprising a fine-tuning member located in the main housing, connected to the wire, and having an inner thread formed in the fine-tuning member; wherein the shaft is provided with an outer thread formed on an outer surface of the shaft and engaged with the inner thread of the fine-tuning member; wherein when the button is rotated, the shaft is rotated and thereby the fine-tuning member moves along the outer thread of the shaft thereby pulling or releasing the wire gradually.