Abstract: According to the principle of equivalent negative stiffness, this paper establishes the equivalent relationship between a top load and an intermediate load in a frame and derives the equivalent load factor formula. Through this factor, an intermediate load can be converted to a top load, so that the stability of a frame can be checked by the effective length gained from code without recomputation. Then, the equivalent force method is proposed. This method can solve the stability problem of a frame under multiple-varying axial force easily with a good accuracy, thusly it can meet the requirements of engineering applications. This paper also proposes the effective length formula for an asymmetry frame with a varying axial force, considering the interaction of columns. The results show that the equivalent force method has good precision and accuracy, and can be used for engineering design and theoretical calculations.