Plane-convex lens or bi-convex lens are commonly used as transform lens while the beam of laser diode array (LDA) is spectrally combined by grating-external cavity method. However, the feedback efficiency of external cavity is rather low for each emitter. For example, feedback efficiency for on-axis emitter is just 80.5% and even decreases to 49.7% for edge emitter. In order to solve the problem, a method that discretely doublet lenses are applied as transform lenses to enhance the feedback efficiency. An equivalent optical setup of traditional spectrally beam combination for LDA is drawn and studied by Zemax optical software. At the same time, feedback efficiency of each emitter will be computed. The imaging characteristic of plane-convex lens is learned by aberration theory and the fact that spherical aberration and coma of plane-convex lens are relative large, which will decrease the feedback efficiency. Funthermore according to primary aberration theory, a discretely doublet is designed by PW method and the doublet is also optimized by Zemax. The feedback efficiency of each LDA emitter whose width is 10 mm is more than 94.2% by using optimized doublet, which is dramatically larger than that of plane-convex lens.The results show that the method is helpful for increasing the coupling efficiency of LDA.