Abstract: In this paper a water cooler for 2D laser diode arrays cooling was presented. The cooler can be manufactured with line cutting method and with very low cost. The thermal resistance of the laser diode with back cooling packaged structure was calculated and the thickness of the submount and the structure of the cooler were optimized by three-dimension finite element method. The calculated results indicated that the thickness of submount, the cooling water channel depth and the fin width have optimum values. The lowest thermal impedance of the water cooler was 0.093℃·cm2/W. The pack age design can meet the needs of heat dissipating for high power laser diode arrays with power density of 500W/cm2 and 20% duty cycle.