To make the laser diode(LD) pump source of laser operate reliably in a wide ambient temperature range without temperature control system, and improve the absorption efficiency and stability of the pump light, the spectral properties of LD and the absorption spectrum properties of Nd: YAG were analyzed, then a new theory and method to design LD pump source with multi-wavelength were proposed, and the absorption length in the gain media was increased to overcome the disadvantage produced by the mismatch between the LD wavelength and the absorption peak of Nd: YAG. A LD pump source with three wavelengths(802.35 nm@25 ℃, 813.15 nm@25 ℃ and 810.95 nm@25 ℃) was designed. The simulation result shows that the pump light absorption efficiency can reach 73.96% under a certain absorption length, and while the ambient temperature changes from -15.7 ℃ to 65.7 ℃, the instability of the output energy is less than 5%. The influence of the absorption length and the doped concentration of Nd3+ in the laser crystal on the pump light absorption efficiency was also simulated.