In this paper， we’ve made a deep research on the far-field propagation of high-power lasers， and a general far-field performance model is derived， which is concerned with multiple factors， including atmosphere turbulence， diffraction effects and etc. Meanwhile for better field application， further amendment suggestions are proposed. Based on this model， we presented the optimal design method and principles by defining index weight factors used as evaluation parameters. The simulation results indicated that both of the beam quality and output power were the key elements that influence the far-field laser irradiance， between them beam quality has a higher weight， about 30%-79%， and needed to be decided first， in fact the ideal case was that beam quality was infinitely close the the diffraction limit. The laser emission aperture weights the lowest， about 4%-24%， and does a little favour to the system performance， so it needs a trade-off design with other functions such as tracking ability. Finally， analysis of the example verify the scientificity of the index weight factors， and verify the validity of the optimal design principles， which have great reference value for engineering design of high power laser systems.