The characteristics of the pumping light distribution in Nd：YAG rod direct side-pumped by diode bars are studied in this paper.Firstly, an approximation but simple analytical solution to the radical distribution of deposited energy in a diode-ba-pumped laser rod is provided based on the characteristics of the Gauss beam and the output of diode laser. Furthermore, the theoretical models of pumping energy distribution in a diode-array-pumped laser rod are made.Then, how the parameters （size of the medium, absorption coefficient of the medium, distance between the laser diodes, and so on） affect the homogeneity of pumping energy distribution in the medium is achieved according to the numerical simulation, and a set of general rules is summarized, and that can offer reference to experimental investigtion.The distribution of pumping light in Nd：YAG rod is measured with CCD camera which is in good agreement with computer numerical simulation. The experiment results are analyzed and discussed. The comparison results illustrate that the theoretical models are correct.Using ray trace algorithm, applying to the diffuse reflection surface medium and applying to multifarious direct side-pumping are the characteristics of the theoretical model.