It is of practical value to study the strength and deformation characteristics of soft clay under cyclic loading and unloading. This study conducted an axial cyclic loading and unloading test of saturated soft clay, using the continuous load one-dimensional K0 consolidation apparatus developed and improved by Chang'an University. The results showed that the stress strain curve of saturated soft clay is hyperbolic during the unloading period. During the reloading period, the curve is hyperbolic when the loading quantity is less than unloading quantity and the curve is straight line and keep hardening when the loading quantity is greater than unloading quantity. The plastic deformation and elastic deformation of test samples produced in the test have linear relation with the unloading level. On the basis of the original constitutive relation, axial circulation loading and unloading constitutive equation have been established using the loading axial stresses, the initial tangent modulus, maximum unloading axial strain and its initial secant modulus as parameters. The reliability of the equations has been verified as well. The hysteresis energy increased with the increase of unloading stress, and there is a good quadratic curve between the consumption of hysteresis energy and unloading stress.