A three dimensional finite element thermal-model is presented for a micro-channel packaged high-power Al-free quantum-well laser diode array(LDA). The influence of working conditions,such as ambient temperature,operating current and duty cycle,on the temperature of the active region is simulated in this paper. And the variety of the output characteristics of LDA working under different conditions is studied experimentally. It is found that longer time to reach steady-state,higher temperature of the active region,greater red shift of peak wavelength and threshold current,lower conversion efficiency and slope efficiency and lower output power are gained when the laser diode array works at higher ambient temperature and duty cycle. The output characteristics of the laser diode array is extrapolated when it works under the conditions of 20 ℃ ambient temperature,20% duty-cycle and 300 A injecting current. The results show that the output power exceeds 300 W and the conversion efficiency achieves 45% and no thermal rollover appears.