Foam draining was important for maintaining normal production of natural gas. However，the water loading in downhole was vulnerable to be contaminated by different chemicals，which made it difficult for us to design and produce highly-efficient foaming agents. In this study，a general analysis method for downhole pollutants was established based on the combination of infrared spectrometer，nuclear magnetic resonance spectrometer and liquid chromatograph mass spectrometer，et al. The main composition and corresponding content of pollutants in water loading from six gas wells in western Sichuan basin were analyzed following this method，and then the source of pollutants was discussed. The results showed that the pollutants content of each well was very different，and the main composition of pollutants was surfactants，inorganic salts and condensate oil. If the pollutants had surfactants，the foaming agents should not contain or only had a little bit of alcohols in order to avoid forming emulsion.Furthermore，using zwitterionic surfactant or nonionic surfactant as the main component of foaming agents，or using acid liquid dissolving inorganic salts could reduce the adverse impact of inorganic salts on foaming operation. In addition，using amine oxide surfactant with long-chain alkyl group as the main composition of foaming agent was beneficial for the draining of wells with high content of condensate oil.