Aiming at bearing-only target tracking for static single station infrared sensor, based on improved trigonometric cotangent relations theorem, assuming that the target moves uniformly and in a straight line, estimating the target track direction with sampling bearing sequence. Further studies on the influence of increasing the span (the interval between the sampling points) on the calculation of the parameters. The track parameters are calculated by means of mean value, and the error is analyzed by the Cramer-Rao lower band, simulation experiments show that increasing the span properly can improve the accuracy to a certain extent.