According to the analysis of the main failure froms and mechanism of plain bearing

although the causes of bearing failure diverse

they all result in destruction of fluid lubriction and contact friction between bearing and shaft. Based on the successful application of the existing diagnostic technique

this paper presents the least Test diagnosis Model as the practical research guideline for the first time

selects contact friction

which is the common charactersic of various failures

as the unique diagnosis base model

and simplifies plain bearing as a gray system model of multiple inputs and single output. The new idea can make not only the research easier but also the result more practical. It can also be applied in fault diagnosis in other regions.