In order to quantitatively analyze the coalescence behavior during the sintering process, the composition and properties of the melt under different alkalinity conditions are calculated by using the thermodynamic software FactSage, and the surface tension and apparent viscosity of the melt under each condition are further obtained according to the calculation results, which are used to calculate the densification coefficient of sinter. The results show that the apparent viscosity of the melt decreases with the increase of alkalinity, and the apparent viscosity has a minimum value (0.114 Pa·s) at a temperature of 1350 °C. The surface tension decreases first and then increases with the increase of alkalinity, and has a minimum value (about 0.32 N) at a temperature of 1300 °C. The coalescence behavior of sinter is driven by the apparent viscosity and surface tension, and increasing the alkalinity is conducive to the coalescence behavior of the sintering process, so that the densification coefficient increases, and the higher the temperature, the greater the densification coefficient. The sintering test shows that in the alkalinity range of 1.85 ~ 2.25, the densification coefficient is related to the porosity of the sinter and the drum index, and the densification coefficient can characterize the sinter strength to a certain extent.