An agricultural machine path tracking method irrelevant to travel speeds, aiming at the path tracking control problem in autonomous operations of intelligent agricultural machines, was presented. The method was robust to changes in travel speeds of agricultural machines. Firstly, the space parameter-driven nonlinear relative kinematics model between the agricultural machine and the tracked path was deduced and the rationality and feasibility of designing control method based on the model was proved. Secondly, the paper dealt with the model by input-state linearization and utilized the optimal control method to stabilize the linear model obtained by feedback linearization, and an agricultural machine path tracking control law irrelevant to travel speeds was then obtained. Finally, path tracking experiments of an agricultural machine were carried out. The experimental results showed that the precision of straight path tracking and curve path tracking were 4cm and 7cm respectively and not affected by changes in travel speeds. Therefore the effectiveness and robustness of the proposed method were verified.