With the wide application of off-axis parabolic mirror in laser processing, how to improve the quality of focal spot becomes a problem to be solved. In theory, the parabolic reflecting focusing approach can eliminate the influence of spherical aberration, and get high-quality focal spot. But the parabolic mirror is one of the most difficult adjustment reflector systems, and it is very sensitive to the optical axis misalignment. So that, the focal power density will decrease rapidly due to the tiny angular misalignment.In order to deepen understanding on the poor off-axis propertites of the parabolic mirror, by geometrical optics and physical optics, the influence of the misalignment angle on the 90° off-axis parabolic mirror focal spot characteristic is studied. The results indicate that the focal spot size is increased and the intensity is decreased greatly because of the tiny angular misalignment. By second-order moments, it is proved a pair of focal lines is produced near the focal plane due to the angular misalignment. It is also proved that the distances between the z planes at which the two focal lines are located and the focus F are equal, and are proportional to the misalignment angle.Because of the existence of the pair of focal lines, the incident Gaussian beam becomes elliptical Gaussian mode, which decreases the peak intensity at the focal plane. The parabolic mirror is very sensitive to the misalignment, and the misalignment angle of several mrad will make the peak intensity at the focal plane drop to the half.