We investigate and analyze the transmission performance of free-space optical (FSO) communication systems at three different working wavelengths, 1.55, 4, and 9 μm, under the combined effects of weather conditions (rain, snow, fog, and haze) and atmospheric turbulence, which is modeled by the gamma-gamma turbulence model. The simulation system of FSO communication based on quadrature phase-shift keying (QPSK) modulation format with a data transmission rate of 10 Gbps is built using OptiSystem software. The results show that the working wavelength has less influence on the transmission performance of the FSO communication system under rainy weather and turbulence conditions. In snowy and turbulent conditions, the short-wavelength laser has a stronger transmission advantage. In fog and turbulence situations, the transmission performance of the FSO communication system is significantly improved with the increase of wavelength. Under the combined influence conditions of haze and medium turbulence, the transmission distance of 9-μm laser is 2.67 and 1.56 times that of 1.55- and 4-μm lasers, respectively, when the bit error rate is 10−9. The results can provide an important reference for the design and optimization of practical FSO communication systems.