Influence of the fine structure mixing rate on the CW end laser diode pumped alkali laser (DPAL) is studied based on the rate equation theory, result shows that the relaxation of the P1/2 and P3/2 energy levels should be described by the upward and downward fine structure mixing rates rather than the thermal equilibrium hypothesis. The influence of fine structure mixing rate on laser threshold can be neglected when it is very large comparing to the spontaneous emission and electronic quenching rates. Not only the match of the pump and atomic absorption width but also sufficient fine structure mixing rate to avoid the absorption saturation is needed for the effective absorption of the pump power. The laser medium length and the temperature can be optimized to achieve high optical-optical efficiency even at low fine structure mixing rate.