Abstract: To determine the relative spatial position of the target to the wellhead,grid north is currently used as the benchmark for North,which positions the ellipsoid into the horizontal plane as the approximate reference coordinate. Because it does not consider the change of magnetic declination with the change of time and space at the measuring points,the conventional calculation of drilled wellbore trajectory exhibits inherent defects and significant errors. In order to solve the problem,True North can be used as the reference North to determine the relative position between the target and wellhead in terms of earth ellipsoid and its calculation principle. It can also be used to determine the magnetic declinations based on international geomagnetism reference field,which considers the changes of magnetic declinations with time and space to calculate the drilled wellbore trajectory by an iterative method. The innovative method can effectively eliminate drawbacks of existing positioning techniques and ensure accuracy and reliability in design and monitoring of wellbore trajectories.