In this paper, several thermodynamic optimization strategies are implemented to tap the potential of multiple water injections in thermodynamic balancing of combined heat and mass transfer process in air humidifier. The all-state of humid air including unsaturation, saturation and supersaturation states are particularly considered. Based on finite difference method, theoretical models are integrated with multiple injection methods. In the off-design analysis, the equipartition process of the driving forces including heat transfer temperature difference and mass transfer pressure difference are full of attention and interest. The research results present that entropy generation minimization is achieved when the values of heat capacity rate ratio equal to unity for all the cases. A larger value of the number of injections will improve the equipartition process of the driving forces, resulting in a reduction of total normalized entropy generation. In addition, it is found that these driving forces can be better balanced with larger number of injections and smaller injection ratio, when the heat capacity rate ratio is constant. Moreover, it is observed that energy effectiveness can be improved and the total normalized entropy generation is diminished as the inlet relative humidity augments, with a minimum normalized entropy generation of 0.0045 emerged at φi = 0.85. Furthermore, it is also highlighted that controlling and adjusting the relative humidity of inlet humid air for the optimization of thermal devices and engineering systems is significant and available, especially referred to the all-state of humid air.