To address the issues of slow clamping force response and deteriorated control accuracy in Electronic Mechanical Braking systems, a composite sliding mode control strategy incorporating a disturbance observer is proposed. Firstly, a phase-based closed-loop control strategy is designed to meet the requirements of different braking stages, enabling precise regulation through a segmented control algorithm. Secondly, based on the traditional exponential reaching law, a composite reaching law is developed by introducing a symmetric Sigmoid function and a power term of the sliding surface. This design enhances the reaching speed while mitigating chattering. Moreover, a super-twisting extended state observer is constructed to estimate system disturbances and feed them back to the controller for real-time compensation. The stability of the proposed control algorithm and observer is rigorously proven using the Lyapunov method. Simulation results under emergency braking conditions demonstrate that compared with the double-power reaching law and super-twisting algorithm, the proposed method improves the response speed by 0.05 s and 0.06 s, respectively, and reduces the average steady-state error by 0.04% and 0.05%, confirming the superior performance of the proposed EMB clamping force control strategy in terms of both response speed and control precision.