The computing formula of the covariance matrix of GDOP (geometric dilution of positioning accuracy) and the factors influencing it are derived by using time difference and target azimuth to analyze the basic principles and positioning accuracy for positioning the static/ moving targets. Using TDOA position method, the general expression and the theoretical curve of positioning accuracy, as well as the relation between the measurement error and the relative geometrical relation between the target and the receiver can be obtained, which can be used to effectively position the static/moving target.