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. <EOS>