Based on combining geometrical and physical optics, the effects of the angular misalignment of the optical axis and divergence angle on the focal power density are analyzed. Ray tracing analysis proves that a pair of focal lines are produced due to the angular misalignment, and diffractive optics proves that incident beam of Gaussian mode brings about reflective beam of near elliptical Gaussian mode. So that, the focal power density is decreased due to the angular misalignment. The parabolic mirror is very sensitive to misalignment, and the misalignment angle of the order of mrad magnitude is enough to decrease peak power density down to the half. The tolerance of the misalignment angle goes rapidly down with the incident beam diameter increasing. When the incident angle has a divergence angle, coma is produced to decrease the power density at focal plane.