The relationship between the beam excursion error and the beam quality β factor of long-term exposure spot was analyzed with and without random wavefront error. The formula to calculate the beam quality β factor of long-term exposure spot with beam excursion error was given. The theoretic analysis and the numerical calculation results indicate that beam quality affected by beam excursion error was independent of the beam quality without beam excursion error, but relative to the criterion of the beam quality β factor and the obstructed ratio of optics system. The beam excursion error request in application was analyzed. The results indicate that the better the beam quality wanted, the smaller the beam excursion error needed.