Large-span steel structures will inevitably cause some relaxation under the influence of actual engineering operations or environmental influences. After the cables are loosened, the force system must be readjusted to a new equilibrium, which will cause the internal forces of the members increase or decrease, and affect the structural safety. The designed cable-truss structure with a span of 6 m as the object, the experiment on relaxation of cable was done. A study was made of the influence to spoke cable-truss structure when different cable was loosened. It is concluded that the effect of relaxation on the loop cable is most obvious, and the loop cable is defined as the relaxation sensitive component, the most unfavorable structure occurs when all the lower radial cables are relaxed. Based on the reliability theory, the influence of different cable tensions on the reliability of the spoke cable truss structure and the sensitivity of different parameters of the structure were obtained by the response surface method and Monte Carlo method, the safety assessment could be performed for the prestress loss of the spoke cable truss structure.