A method for calculating navigation parameters via RTK— DGPS fusing inertial sensor was proposed in order to improve the navigation precision of agricultural vehicle. The key for calculating the lateral deviation was the compensation of the lever-arm effect error via the geometric transformation with the attitude angles form the inertial sensor. Test results showed that the mean error compensated by this method was 0.08m when the pitch and roll was existed. Benefit from the compensation, we got more accurate lateral deviation by the enhanced positioning accuracy. The error of the heading deviation calculated by the inertial sensor will be larger when there is magnetic interference around the working environment. Therefore, another method to calculate the heading deviation was proposed, which is using least-square method to fit the travel path with the dynamic positioning points measured by RTK—DGPS. Test results showed that when the vehicle was steered along a straight line, a circle and a curve, the mean errors were 0.9636°, 3.6418°and 2.7562° between two methods which were measured and calculated by RTK—DGPS and the inertial sensor. So it was proved that the heading deviation could be calculated by RTK—DGPS effectively. Two methods can enhance the adaptability of the system, the heading deviation can be got by RTK—DGPS instead of the inertial sensor under the environment with magnetic interference.