Abstract: A linear degradation humidifying model for drug stability experiment is introduced. This new humidifying model is presented as: H_r=-1Mlnexp(-MHr,0)-［exp(-MH_r,0)-exp(-MH_r,m)］tt_m. Where H_r is the relative humidity; t is the time; H_r,m and t_m are the final relative humidity and time of the experiment, respectively. M is humidifying constant used in the humidifying program. In the new programmed humidifying model, a linear relationship between the content function of drugs and the relative humidity is obtained, the degradation of drugs can be more uniform within different humidity ranges and the experimental results are more accurate than those in the reported linear humidifying model. The stability of penicillin potassium, as a solid state model, was investigated by the linear degradation programmed humidifying and the exponential heating experiments. The results indicated that the kinetic parameters obtained by the linear degradation programmed humidifying and the exponential heating models were significantly more precise than those obtained by the linear programmed humidifying and the reciprocal heating models.