A model for partially coherent light propagation suitable for synchrotron soft X-ray is established based on statistical optics theories. We use mutual optics intensity to describe the partially coherent light, and sum the discrete wavefront considering reasonable approximation. The coherent properties after propagating a distance are numerically analyzed with the model. The influences of propagation distance, coherent length of source and source size on the coherent properties at sample position are studied with the established model. Meanwhile, coherent diffraction experiments are carried out in the soft X-ray interference lithography branch beamline (BL08U1B). Simulations on the experimental results are performed according to the model. The results show that the simulations coincide with the experimental patterns. The model can be used for the design of the synchrotron beamlines with high coherent properties.