Evaporation duct has strong impacts on the performance of marine radar and communication systems. Evaporation duct channel is a promising channel for marine high capacity data transmission. Wave propagation models and evaporation duct models are well studied, and statistical features of duct channel has been observed. However, the work on the physical model of small-scale characteristics of evaporation duct channel is seldom reported. So we make efforts to establish a physical model on small-scale characteristics like spatial correlation using WM sea surface model. In this paper, the vertical space diversity feature of evaporation duct communication channel is studied both from numerical simulation and experimental observation. And the results could be applied in marine MIMO communication systems.