A new direct-contact dehumidifying economizer was developed, in which the packed mass was set for the recovery of waste heat in the flue gas exhausted from gas-fired boiler. An investigation of the heat and mass transfer process in the packed mass section and the design model was carried out to build and improve the economizer designing method. A set of differential equations were developed in which the flue gas temperature tg was used as an independent variable and the Lewis Number Le, the wet specific heat CH of mainstream flue gas and CHi of the flue gas on the phase interface between the flue gas and water were included. The differential equation set can be used to calculate the parameter distribution in the heat and mass transfer process in the packed mass section and the packed mass section height. The calculation results agree well with the experimental measurement. This has proved that the calculation method developed can be applied in the design and analysis of the new direct-contact dehumidifying economizer.