In this paper, the laser beam attenuation by sand/dust storms is experimentally investigated under controlled lab conditions for 5 laser wavelengths, i.e., 405 nm and 532 nm in visible band and 850 nm, 1064 nm and 1550 nm in near infrared band. It is found that the attenuation coefficient of the laser links exponentially decreased with the increase of the visibility of dust/sand storms mimicked in a glass chamber with mixed-size sand samples. The longer the laser wavelength, the lower the attenuation coefficient under the same visibility, e.g., when the visible range is 0.1 km, 0.5 km and 1 km, the attenuation coefficient of the laser links with the wavelength 405 nm is 174 +/- 10.9 dB/km, 33.7 +/- 4.7 dB/km and 17.7 +/- 1.6 dB/km, while the attenuation coefficient of the laser links with the wavelength 1550 nm is 126 +/- 10.7 dB/km, 23.7 +/- 10.8 dB/km and 9.6 +/- 2.5 dB/km, respectively. In addition, it is found that the attenuation decreases with increasing effective particle size under a given visibility. Furthermore, based on the experimental results and Kruse model, an improved model for the calculation of laser beam attenuation by sand/dust storms, which is a function of both laser wavelength and effective particle size of the sand/dust storms, is proposed.