The influencing relationship equation of humidity distribution on the characteristics and extent of cross diffusion was proposed based on the nonequilibrium thermodynamic theory. With the parameters of temperature, humidity and contaminant gas concentration, a further study on the relationship was employed. It was showed that the temperature grads and the contaminant gas mass grads were less than zero when the humidity grads and additional diffusion coefficient were both positive/negative, otherwise the two grads were more than zero. Moreover, the higher the initial temperature and contaminant gas concentration were, the greater the absolute values of temperature grads and contaminant gas mass grads were, with the same humidity grads and additional diffusion coefficient. Whereas the influence of initial temperature was limited, and that of initial contaminant gas concentration was obvious.