In the precision agriculture (PA), an agricultural vehicle navigation system is essential and precision of the vehicle path tracking is of great importance in such a system. As straight line operation is the main way of agricultural vehicles on large fields, this paper focuses on the discussion of straight-line path tracking control methods and proposes an agricultural vehicle path tracking algorithm based on the optimal control theory. First, the paper deduces a relative kinematics model of agricultural vehicles based on lateral deviation and heading errors between vehicles and paths. And then a linear quadric (LQ) optimal controller is introduced to improve the control precision and other performance index, such as stability and fast response. The stability of the controller at different speeds is also discussed and the stability condition according to Lyapunov stability theory is proved. Finally, the feasibility of the control algorithm is verified by a series of experiments with a combine operating on a road. The results show that the algorithm proposed in the paper yields satisfactory effects on the straight-line path tracking of agricultural vehicles. At slow speeds, the range of lateral position deviation of the straight-line path tracking is approximately from -0.08m to 0.12m and the mean value of the lateral position deviation is 0.05m.