High power semiconductor lasers have found increasing applications in many areas. The junction temperature rise may not only affect output power, slope efficiency, threshold current and lifetime, but also cause spectral broadening and wavelength shift, which makes thermal management one of the major obstacles of pump laser development. It is more and more crucial to carry out thermal design and optimization. This paper studied the steady and transient thermal behavior of a single-bar CS-packaged 60W 808nm laser in continuous-wave state using numerical analysis method and experiments. Time constants, as well as thermal resistance and its compositions were quantificationally analyzed. Moreover, high power semiconductor lasers were produced based on the thermal design and optimization.