A bicycle brake assist, comprising a main body, a traction device, two control cables, a rear wheel brake cable and a front wheel brake cable, wherein said traction device has a moving block and a rotary piece pivotally connected with each other, characterised in that said main body has a first surface and a second surface, said first surface being provided with a chamber which is depressed toward the interior of said main body; when said traction device is installed in said chamber and said control cables, said rear wheel brake cable and said front wheel brake cable are connected in place, a cover plate is combined with said first surface to enclose said chamber; two slot-shape grooves (104, 111) opposite to each other are separately provided on the center of said cover plate and the center of said main body; two first through holes communicating with said chamber and mutually adjacent to each other in left and right direction are provided on the upper side of said main body; a second through hole and a third through hole communicating with said chamber and adjacent to each other in front and rear direction are provided on the lower side of said main body; said moving block has a front end portion and a rear end portion, and a central axle projecting out of said front and said rear end portions (211, 212) is provided at the center of said moving block; when said moving block is placed into said chamber, said central axle is engaged in said slot-shape grooves (104, 111) separately of said cover plate and said main body, so as to keep said moving block in position; by mutual constraint of said central axle and said slot-shape grooves (104, 111), said moving block is able to conduct limited up-down movement in said chamber; further, said moving block has two first engagement holes provided on the upper edge thereof and spaced apart equally from the center of the upper edge, and a second engagement hole provided on the center of the bottom edge; escape chamfers provided on the bottom edge of said moving block are separately provided on both sides of said second engagement hole; said rotary piece has a pivot slot provided at the center thereof and extending upward, and a third engagement hole provided at the bottom portion thereof; said rotary piece is pivotally connected with said moving block in the chamber of the main body by inserting the central axle through said pivot slot of said rotary piece; under pivot connection with said moving block, said rotary piece abuts on said front end portion of said moving block so as to be adjacent to said cover plate; each control cable has a control wire encapsulated therein, said two control wires are coupled separately to brake levers at one end and locked separately in said two first engagement holes of said moving block at the other end after being pulled into said chamber through said two first through holes of the main body; said rear wheel brake cable has a rear wheel brake wire encapsulated therein, said rear wheel brake wire is coupled to said rear wheel brake at one end and locked in said second engagement hole of said moving block at the other end after being pulled into said chamber from said second through hole of the main body; said front wheel brake cable has a front wheel brake wire encapsulated therein, said front wheel brake wire is coupled to the front wheel brake at one end and locked in said third engagement hole of said rotary piece at the other end after being pulled into said chamber from said third through hole of the main body.