The non-linear and time-varying mathematical model of the anti-lock braking system (ABS) is established, and the application of sliding mode variable structure controller (SMVSC) in ABS is implemented. Two kinds of estimation algorithms of the optimal slip ratio realized by recursive least squares are proposed: one based on the mathematical model of cohesion coefficient vs. slip ratio curve, the other based on the shape of this curve. The simulation results show the feasibility and validity of SMVSC based on optimal slip ratio.