Abstract: A differential robot is a typical mobile robot widely used in storage, agriculture, and other industries. The motion control of differential robots, including longitudinal and lateral control, is a current research hotspot. To date, researchers have not paid much attention to the interaction between longitudinal and lateral control of differential robots. However, the conflict between the ability to track the reference path and maintain the longitudinal speed at its maximum value is a critical issue that limits the operational efficiency of the differential robot. To solve this problem, a mapping relationship between the longitudinal speed and the turning curvature is analyzed. The mapping relationship is established when the maximum value of the longitudinal speed is known, i.e., the feasible upper limit of the longitudinal speed that can guarantee the steering ability of the differential robot is inversely proportional to the curvature of the trajectory. From this mapping relationship, a speed-adjusting method is proposed based on the preview information. This speed-adjusting method consists of two steps. First, the smaller value between the upper limit of the feasible longitudinal speed in a certain preview distance and the set value of the longitudinal speed are taken as the desired longitudinal speed. Second, a control law is established based on the deviation between this desired and current longitudinal speed. Additionally, a path-tracking method that cooperates with the above-mentioned speed-adjusting method is proposed. The theoretical basis of this path-tracking method is a nonlinear model predictive control. The prediction model used in this control method is derived from a kinematic model with longitudinal speed as a time-dependent parameter. Finally, a differential robot motion control system is formed based on speed adjusting and path tracking. The simulation and experimental results show that the proposed motion control system can actively adjust the longitudinal speed when the set value of the longitudinal speed of the differential robot is high and ensure high accuracy of path tracking control. Furthermore, the absolute value of the displacement and heading errors does not exceed 0.0499 m and 0.0726 rad, which are reduced by 97.57% and 45.04% compared with the motion control system without speed adjusting, respectively.