In compiling the map of China metamorphic petrology, the author has re- viewed more than thirty ordinary petrochemistry methods in regard to their character and application, and he has noticed there are two erroneous tenden- cies in the application of these methods both in our country and abroad, one of them only used one or two petrochemistry methods in restoring the original rocks, the other throwed doubt on the effectiveness of all petrochemistry met- hods. The author analysed the causes which produced above two erroneous tendencies. Two principles of petrochemistry methods are discussed in this paper and considered essentially to be true. The first is that the regional metamorphism is equichemical, i. e. there is no chemical change in the rocks in the processes of metamorphism, except their HBO and CO2 content. The second is that the chemical composition should be different between the various genetic types of original rocks either in their major elements or minor elements, and even in the relationshipes of them. For the correct use of the petrochemistry methods the author puts for- ward four items of suggestion, in which should be especially emphasized is that oue should pay attention to the chemical peculiarities of various volcanic-sedi- mentary rock types and their distributive characters in various petrochemis- try diagrams. In addition, It is necessary to have an intimate understanding of selected methods including their theoretical foundation,applicability, advantages and their defects.