1. A constructing method of an analog circuit for establishing testing procedures by applying fault classification tables of said analog circuit, wherein said constructing method establishes an analog testing tool by constructing a decision tree, said constructing method comprising the steps of: establishing said fault classification tables of test points of said analog circuit, wherein each one of said fault classification tables comprises a plurality of fault groups established by at least one failure mode of said analog circuit; establishing an inference state according to each one of said test points, wherein said inference state comprises at least one of one or more suspect groups to determine a selected test point from said test points; generating a new inference state of said selected test point to construct said decision tree by using an inference state entropy of said inference state and said fault classification tables of unused test points, wherein said new inference state comprises at least one of said suspect groups and said new inference state is used to determine a next test point which is next to be tested after said selected test point; and terminating operations of said constructing method when all suspect group entropies of said suspect groups in said new inference state are equal to a preset value, or when all said fault classification tables of said test points have been used.