A wheelchair safety device comprises a brake lever, four link levers, an upper brake plate, a brake wheel, a lower brake plate, a parking wheel, and an angularly adjustable mounting plate. One end of the first, the second and the third link levers is respectively articulated at the brake lever, another end of the first and the second link levers are articulated with one end of the fourth link lever. Another end of the fourth link lever is articulated at the upper brake plate and another end of the third link lever is articulated at the lower brake plate. One end of the upper and the lower brake plates is respectively articulated at the angularly adjustable mounting plate, and another end thereof is mounted with the brake wheel and the parking wheel, respectively. The brake wheel and the parking wheel respectively act on the wheelchair.