In a practical high power laser system, it may happen that all optical elements do not share the same axis, some optical components may show certain astigmatism, and laser beams may be deliberately arranged to incident at non-zero angles to the optical axis. In this work, using the 4×4 optical matrices, the method for locating the off-axial ghost points (or ghost lines) of various orders in high power laser system have been given. In addition, practical criterions of specifying real and virtual ghost points (or ghost lines) have been put forward.