Material internal parameters, such as internal quantum efficiency, internal loss, transparent current density mode gain and so on, were obtained by P-I test for different cavity length 808nm semiconductor laser diodes. The relation between cavity facet reflectivity and power conversion efficiency was analyzed, and the relation curves were acquired by the design of cavity facet reflectivity according to these internal parameters. Then, we did cavity facet coating experiments. Experimental results are well consisted with the theoretical prediction through comparison. The maximal power conversion efficiency can be achieved through the design of cavity facet reflectivity, and the semiconductor laser diodes can work in an optimal condition.