Based on the model of cement hydration， considering the dynamic evolution law of porosity and ion concentration of pore solution in the process of cement hydration， the relationship of water binder ratio， cement components with electric conductivity of hardened cement paste was established， and a dynamic calculation model for electric conductivity of hardened cement paste was proposed. The results show that the proposed model can calculate the electric conductivity of hardened cement paste at different ages and the overall calculation error is less than 10%， especially the calculation error of electric conductivity of hardened cement paste at 28 d is less than 5%. With the increase of cement hydration age， the ion concentration of pore solution first increases and then stabilizes， while the porosity decreases continuously with time. The combined effect of the two causes makes the electric conductivity of hardened cement paste decrease with the increase of cement hydration age. With the increase of water-binder ratio， the ion concentration and electrical conductivity of pore solution decrease， but the conductivity of the hardened cement paste increase due to the increase of porosity.