Cloud contamination of greenhouse gas satellite data is investigated by using O_2-A band, the scattering effects on CO_2 retrieval from optical path-length is analyzed and a new method for solving cloud contamination problem is proposed. The results show that 83.58%, 9.70%, and 6.72% L2 data are contained in clear, cloud, and undetermined scenes. The CO_2 retrieval of these clear scene and cirrus scene appears highly agreement with L2 product, but the other CO_2 retrieval is generally lower than L2 product. Cloud screening in O_2-A band has its own advantage but also existing insufficiency like the misidentifying of thin cirrus. Fortunately, the scattering correction method through optical path-length is an effective supplement to O_2-A band cloud screening as it can improve CO_2 retrieval bias due to cirrus scattering, combining them to account for cloud contamination is a promising method. Simultaneously, undetermined-I scene should be pay more attention since these undetermined scene contained abundance scene covered by cirrus.