The free space propagation property of broadband laser beam in Fresnel diffraction area is studied based on Huygens-Fresnel diffraction integral. The relationship between modulation depth, bandwidth Δλ and Fresnel number is obtained. It is found that the uniformity of the beam is improved to a certain extent for an appropriate bandwidth. The bandwidth is widened, and the beam is more uniform, for Δλ > 2λ0/F. Fresnel diffraction disappears completely when Δλ = 2λ0/F is fulfilled. For Fresnel number in a certain range, the modulation depth of broadband laser beam oscillates with the Fresnel number, with the main and secondary maximums of the oscillating curve appearing for odd and even Fresnel number respectively. Its minimums exist for Fresnel number F=2k±1/3 (k=1, 2, 3 …), and meanwhile the uniformity of the laser beam is the best.