Abstract: The relationship of temperature and stress to molecular motion was approved according to the free volume theory in this paper.On the basis of time-temperature superposition,the principle,shift formula and factor of load superposition were represented,which could accelerate creep test of geosynthetics and predict its long term behaviour.The laboratory creep test of geogrid RS50 were analyzed.Smooth primary curves were formed with the help of creep test curves at different load levels through load superposition method.Stress-strain curves at the end of designed life and creep reduction factor of geogrid RS50 are determined,which could be used as reference for engineering design.