Mathematical modelling of salt gradient solar ponds (SGSP) is needed for predicting their performance, performing parametric studies, and above all for design purposes. In the present work, one dimensional simulation model for a SGSP is developed in order to simulate the transient behaviour of a SGSP in a southern part of Tunisia. This dynamic model, rakes into account top losses and heat losses to the ground and uses hourly meteorological data for the site of El Bibane in the south of Tunisia. The governing differential equations are solved by the finite difference method. A computer program is proposed in order to perform the calculation. The results show that solar ponds (SP) are reliable as solar collector and storage systems which can provide the needed energy for thermal desalination process.