The data from natural gas are relatively simple, including two types only, namely, components and isotopes of gas. But these data must have contained many processes for gas from source rocks to reservoirs, such as generating, expulsing, migrating, accumulating and alteration, which may affect the components and isotopes of natural gas. Based on a lot of gas data and many studies, the main factors for controlling geochemical characteristics of natural gas were discussed in the paper, taking natural gas in the Tarim Basin as an example. The data showed that the components and isotopes were mainly controlled by source type and maturity (being called "source controlling" by some reseachers), filling process and secondary action after accumulating. The recognition of controlling factors on the components and isotopes of natural gas might make us to deepen understandings the data of natural gas. In addition to source type and maturity (the source controlling), filling process and secondary action after accumulating must have been important factors for geochemical characteristics, which would be helpful to study and exploration of natural gas.