Based on the method for evaluating oil and gas geochemical anomaly and the principle for formulating its parameters,this paper points out that the formulated parameters should not only have definite geological and geochemical implications but also accord with a certain mathematical principle as well as the optimization criterion.On such a basis, the paper deals with the attributes of anomaly and background as well as methods for their determination,describes roughly the criteria for judging oil and gas anomalies,emphasizes the importance of the correlation between composite anomalies and variables as well as the extraction of associated information.