Abstract: Based on the assumption that the characteristic parameters of spring and dashpot of viscoelastic element are a function of time, double-variable parameter Maxwell models were established. Their relaxation functions were obtained by resolving the variable parameter constitutive relation. The relation between the relaxation function and the empirical KWW function was discussed, and a new empirical model for stress relaxation was proposed. The parameter determination method was established by the principle of the least square. The long term stress relaxation test of glass-fiber reinforced plastic (GFRP) composites was carried out, and the proposed model was verified through the experimental data. The research results show that the proposed model curve and experimental curve approximately coincide with each other, and the correlation coefficient reaches 96% or more. It shows that the model is suitable to describe the viscoelastic relaxation properties of GFRP composites.