The Beidou positioning system (BDS) can achieve centimeter level positioning, and has been widely used in agricultural machinery navigation systems. However, a simple satellite positioning system has some limitations, such as being vulnerable to external occlusion and low data frequency. Therefore, integrating inertial components and developing integrated navigation technology in navigation systems is an important trend. Considering the high cost of high-performance inertial navigation, which is not conducive to promotion and application, the use of low-cost inertial navigation and BDS was studied to constitute an integrated navigation system in agricultural scenarios. In order to improve positioning accuracy and solve the problem of error divergence caused by BDS interruption, zero speed correction was designed. At the same time, the causes and sources of unobservable heading angle errors in the BDS/INS integrated navigation system were analyzed, and a heading constraint method was designed. When BDS information was available, dual antenna heading angle information was used to suppress the accumulation of heading angle errors. When BDS was interrupted, zero angular velocity correction was used. Field experiments verified that when BDS was available, zero speed correction can improve the accuracy of position, velocity, and horizontal attitude by more than 20%, 40%, and 15%, respectively. Heading constraints can improve the accuracy of heading angle by more than 90%. When BDS was interrupted, zero speed correction can improve the accuracy of position and speed by more than 90%, horizontal attitude accuracy by more than 80%, and heading constraint can improve the accuracy of heading angle by more than 40%. 