High power semiconductor laser arrays have found increasing applications in pumping of solid state laser systems. Spectral width is one of the key factors of laser array products. Increasing the spectral accuracy by reducing the spectral width of the pump diode can improve the laser system compactness,efficiency,power,and beam quality while reducing thermal management cost in the system. The mechanisms of spectral broadening in high power semiconductor laser arrays are studied for the first time by numerical simulation and by means of spatial spectral mapping and scan acoustic microscope(SAM). The broadening of a laser spectrum is often the result of the appearance of a shoulder/tail on either or both sides of the spectrum or sometimes a double-peak or even multiple peaks occurs. It is concluded that the shoulder/tail appeared on the longer-wavelength side of the spectrum is normally caused by thermal effect while the shoulder/tail on the shorter-wavelength side of the spectrum is usually the result of thermal-stress built in the laser array. Based on the mechanism of spectral broadening in high power semiconductor laser arrays,the strategy of controlling spectral broadening is presented in this paper.