A hidden braking device installed on an adjustable walking aid has a connecting unit installed in the walking aid. The connecting unit has an upper shaft connecting to a braking grip, a lower shaft connected to a braking unit, an adjusting seat fixed to the lower shaft for the upper shaft to penetrate, a moving member slidably disposed in the adjusting seat for propelling the upper shaft, and a fastening unit disposed out of the walking aid, corresponding to the moving member. In adjusting the height of the walking aid, the fastening unit utilizes its magnetic attraction to control the moving member. The moving member is influenced to engage with one fixing slot defined on the upper shaft. The height of the walking aid and the length of the connecting unit are concurrently adjusted, which increases the operational convenience.