Abstract: The load-deflection behaviors of constant force spring are analyzed and discussed systematically by using the principle of potential energy subsections. Then, an example is given for comparing their load-deflection behaviors when constant force spring is frontwound and backwound respectively. In addition, the effects of the diameter of drum on the spring's characteristics are obtained. The results show that the principle of potential energy subsections can cause a result quite accurately and be used as a guide for designing a constant force spring.