The influence of weather can deteriorate the performance of free-space optical communication system. Based on the Mie theory and two kinds of raindrop spectral models, the laser attenuation induced by rain with different raining rates is calculated. The laser attenuation due to visibility is calculated using Kim model and AlNaboulsi model. The performances of free-space optical communications in rain and fog weather are studied. The relation between the bit error rate and the transmission range, as well as that between the bit error rate and the transmission power, is analyzed in different raining rates and rain or fog weather condition. The results show that raining rate influences the transmission ranges, and the attenuation induced by fog is much bigger than rain, which leads to a shorter transmission range. To guarantee the normal work of the system, the transmission power can be appropriately increased.