The invention discloses a hand brake operation unit, a braking system, a mountain bike and a bike. The hand brake operation unit is characterized in that the hand brake operation unit comprises a handlebar frame, a first part, a second part, handlebar sleeves and elastic resetting mechanisms; the first part and the second part are fixed to the ends of the handlebar frame through pulling plugs; the first part and the second part are sleeved with the handlebar sleeves; the elastic resetting mechanisms are arranged between the handlebar frame and the handlebar sleeves; a first reinforcing part of the first part and a second reinforcing part of the second part are coaxially arranged and a mounting channel is formed between the first reinforcing part and the second reinforcing part; and after the handlebar sleeves are rotated, a first pinion and a second pinion rotate in the same direction and the thread screwing directions of the first pinion and the second pinion are opposite, so that a steel wire rope connecting piece and a wire jacket connecting piece between the first pinion and the second pinion move in opposite directions, and braking action is generated. The hand brake operation unit is operated by means of wrist power of people, the traditional mode that braking operation is conducted through pulling with fingers is changed, and the hand brake operation unit better accords with human engineering.