An electricity-water co-generation system and method, the method comprising: step 1, heating saline water by spent steam, crystallizing a calcium magnesium compound in the saline water, then filtering the saline water; step 2, distilling the descaled water, and discharging strong saline water; step 3, converting the heat energy of the saturated steam into electric energy, dividing condensed water generated by releasing heat energy of the saturated steam into two parts, one part being output as fresh water; step 4, heating the other part of condensed water, and converting the heat energy of the overheated steam into electric energy, then feeding spent steam to heat the saline water and distilling the descaled water at a high temperature. The system comprises a descaling device, a mixed heat exchanger, a residual heat generator and a thermal power generation device, and the working process of the system is accordance with the method.