The design or assessment method to identify the safety of all the materials or components in accordance with the specific service environment is called design or assessment method for applicability. The assessment method for material applicability is set forth in Standard API 579.The most important step in applicability design is to simulate the practical conditions on the site as close as possible. However, limited by material, manufacturing level and cost, etc., it is hard to simulate the real environment in the laboratory test. Therefore, the testing condition and method should be designed scientifically to allow the testing result to close to the practical condition as much as possible. At present, as for the design of the gas component, the content of each component during the test is designed based on the principle of partial pressure equalization of corrosive component according to the actual pressure and the gas components on the site. But the author assumes that the principle of dissolved amount equalization for the corrosive components in the solution shall be adhered to in the process of applicability assessment. The paper mainly introduces the method to calculate the partial pressure of each component, the model to calculate the solubility of the corrosive components in the solution, and finally gives the example of calculation and points out the problem existing in the partial pressure equalization principle.