A hidden braking device includes a brake handle, a brake member controlling a wheel set, and a connecting member disposed in a main frame having telescoping upper and lower frames. The connecting member has an upper stem connected to the brake handle and having fixing notches, a lower stem connected to the brake member, a locking set disposed on the lower frame and mainly including a locking unit, and an adjusting set connected to both stems. The adjusting set mainly has an operating unit on which a first spring unit is disposed, an engaging case and a positioning case disposed relative to each other, and an elastic unit located between both cases and engaged with the fixing notches. The first spring unit pushes the operating unit while loosening the locking unit to thereby position the engaging case and adjust a full length of the main frame quickly.