Abstract: Using the formula of the distance between two points , the deformation-displacement equation of axial force bars with small transfiguration is educed from the definition of bar's deformation. This equation has a pithy form, is easy to calculate, can be applied to solve the problems of small deformation of either linear or nonlinear materials, has no need to distinguish between statically determinate and indeterminate structure, can handle pedestal subsidence easily, thus is suitable for use to analysis truss structures widely.