In this paper, a speed tracking controller is designed for the All-terrain vehicles. The method of feedforward with state variable feedback based on conditional integrators is adopted by the proposed control algorithm. The feedforward is designed considering the influence of the road slope on the longitudinal dynamics, which makes the All-terrain vehicles satisfy the acceleration demand of the upper controller when it tracks the desired speed on the road with slope varying greatly. The road slope is estimated based on a combined kinematic and dynamic model. This method solves the problem that road slope estimation requires an accurate vehicle dynamic model and are susceptible to acceleration sensor bias. Based on the vehicle dynamic model and the nonlinear tire model, the method of conditional integration is used in the state variable feedback, which considers the saturation constraint of the actuator with the intention of preventing the divergent integral operation. The control algorithm proposed in this paper can meet the requirements of vehicle speed tracking by controlling the engine driving torque and EHB brake fluid pressure. In the end, the speed tracking control algorithm is verified by the real vehicle tests, which makes the All-terrain vehicles track the desired speed effectively and acquire the desired acceleration. Furthermore, it ensures that the tracking error varies within the range of ±2km/h, meeting the requirements of national vehicle test standards.