The invention provides a fuel economizer with sliding and automatic sliding forbidding functions. The fuel economizer comprises an automatic sliding forbidding mechanism and an overrun clutch; the automatic sliding forbidding mechanism comprises a clutch bearing seat, a clutch shell, a meshing wheel, a compression spring, a shifting fork, an air cylinder, an ejector rod, a return spring, a push rod and a meshing pin roller; the clutch bearing seat, the clutch shell, the meshing wheel and the shifting fork are installed on a spindle, the inclined surface of the ejector rod is in contact connection with the push rod, and the return spring and the meshing pin roller are installed on the push rod; the overrun clutch comprises a meshing pin roller, a large drum wheel and three sets of star wheels, the star wheels are installed in the large drum wheel and are fixed to the spindle, a sliding pin roller is installed on the star wheels, and the surface, combined with the meshing pin roller and the sliding pin roller, of the large drum wheel is provided with a regular concave surface. In the sliding process of a vehicle, connection between the vehicle and a driving system is automatically cut off, so that the sliding distance is increased, energy is saved, the function of automatically prohibiting sliding according to working conditions is achieved, and the original vehicle braking performance is improved.