The character of the pumping light distribution in Nd∶YAG rod gain medium direct side-pumped by diode bars with half-ring form are studied in this paper.An analytical solution to the radical distribution of the pump light in the rod gain medium is provided based on the different parameters.Through the method of ray trace the simulated numerical calculation and the variation laws are made with the software Matlab,the influence to the distribution of pumping light is analyzed with the parameters of the pumping-structure and shows the laws of the influence,then analyses the reason why the difference occurs,and in the end the principle about how to take those parameters have been suggested.The theory can offer reference to design the solid state laser rods side-pumped by laser diode.