During postulated severe accidents of a light water reactor, a debris bed may form in the lower head of the reactor pressure vessel due to Fuel-Coolant Interaction (FCI), and re-melt into a molten pool if the debris bed is uncoolable. The debris bed remelting is therefore an important process in a severe accident scenario. To predict the dynamic process of debris bed remelting, a computer program is developed in the present study using lumped-parameter models. The melt in the lower head is split into different zones of molten metal, molten oxide and solid debris particles submerged in molten pools. Correlations are employed to calculate the heat transfer within each zone and between zones. The developed lumped-parameter code is employed to calculate the COREM experiments. The comparison of the simulation results with the experimental shows a reasonable agreement for melting processes of single-material and two-material debris beds. The code is also used to investigate some factors which may affect debris bed remelting, such as internal heating power, volume ratio of components, and thermophysical properties.