A digital intelligent monitoring system based on CAN bus was constructed. Based on CAN 2.0B frame structure, the principle and method of the CAN bus application-layer protocol constitution were addressed by analyzing the message identifier of extended superframe. The CAN bus application-layer protocol of monitoring system suited for cotton-picking machine was proposed. This protocol was applied on the prototype of cotton-picking machine. The experiment results showed that the proposed protocol not only met the need of intelligent monitoring system, but also be characterized by good portability and scalability. The research results can also be applied in other agricultural and engineer machinery.