Along with the intelligence and automation improving in agricultural machinery, the application of steering-by-wire technology in agricultural vehicles such as tractors has received concerns and researches. In order to guide the study of hydraulic steering-by-wire of tractor and to shorten the development cycle, a co-simulation model was built based on the analysis of hydraulic steering-by-wire system control algorithm and structure. The hydraulic system and full vehicle model were built on AMESim software platform. Meantime, PID, fuzzy PID and fuzzy immune PID control modeling were constructed by using Simulink software. While Visual C++ 6.0 was chosen to realize interface communication, the angle response and angle following co-simulation were completed with angle transmission ratio 1; as well as the co-simulation of yawing angular velocity response and centroid side-slip angle response under the speed of 15 km/h, the steering wheel Angle 180°, angle transmission ratio 9. The experimental results show that step response time achieves 0.272 s with fuzzy immune PID control, following error of 1.182°, 3% overshoot amount of the yawing angular velocity response, and 0.85°/s steady-state value of centroid side-slip angle response, they both prevail over the performance of common PID and fuzzy PID. In conclusion, co-simulation maintains a strong reference value, and an ideal control effect is able to achieve while fuzzy immune PID control are applied into hydraulic steering-by-wire system.