Abstract: Off-target distance is a key index for measuring quality of borehole trajectory,of which the key is to find out solution to coordinates of target points.By defining the normal direction of target planes,a general target plane equation that can be used to describe arbitrary posture of targets is deduced.In this paper,the method of determining position of target points on measured sections is studies,as well as the interpolation formula commonly used for survey calculation is presented.Based on this,the method for calculating coordinates of target points and off-target distance is obtained,as well as two representing methods for off-target distance are given.In addition,the method for identifying whether or not hit the target is discussed.Research shows that the equation of target plane,the interpolation of borehole trajectory,and the calculating method for off-target distance have universality and can be applied to calculate off-target distance of various well types and target plane.However,horizontal and plumb targets are special cases.An error estimation model is proposed for linear interpolation method usually applied on site.It is not recommended to use the linear interpolation method since it has much error.