A salt gradient solar pond with porous medium bed has been paid more and more attention, due to potential positive effect of medium on the thermal performance of salt gradient solar pond and also the delay action salt diffusion. Stability of salt gradient pond depends on the maintenance of the salt gradient. In this paper the effect of the porosity porous material on the salinity under the constant Lower Convective Zone (LCZ) temperature is experimentally studied, and the cleaned and screened coal cinder used as the adding porous material. Under the same salt gradient conditions, the salinity development of LCZ with porous medium with porosity of 61%, 65.5%, 67% and 74% is respectively studied, the blank experiment without adding any porous material was used comparison, and according to the experimental data, the salt diffusion coefficient in each depth was calculated by numerical Experimental results show that within the experimental temperature range, the porous medium can delay the salt diffusion upward. signs show that the porous material seems to have the absorption effect on salt. The smaller porosity the porous medium is the slowly salt diffuses. (C) 2015 Elsevier Ltd. All rights reserved.