A novel cycle model of the capacitive salinity/heat engine mainly consisting of nano-porous super-capacitors is established for harvesting mixed free energy caused by salinity difference between the river water and the seawater, and the thermal energy due to the temperature difference. The heat engine is charged and discharged in the cycle of a low temperature brine and a high temperature fresh water, respectively. The analytical expressions of the cyclic work output and efficiency are given. General performance characteristics of the capacitive salinity/heat engine are analyzed. The temperature of the heat reservoir and salt concentration of seawater are optimized to improve the efficiency of the capacitive salinity/heat engine. The optimal selection ranges of several main parameters are provided. When the Stern distance is equal to 1 nm and the charging voltage is equal to 3.5V, the energy conversion efficiency of the capacitive salinity/heat engine is about 40%, while the pure thermal efficiency in the cycle is about 16.9%. The energy conversion efficiency of the capacitive salinity/heat engine is significantly larger than that of the capacitive heat engine. The results obtained can facilitate the application of this technology to real life, which can reduce the consumption of fossil fuels.