To meet the requirements, requested in infrared-laser testing techniques, in spectral range and peaks precision of multi-spectral source, we present a method for preparing multi-wavelength infrared laser diode with a high precision, and design a packaging structure which can integrate four kinds of laser chips with wavelengths 860 nm, 905 nm and 1064 nm (pulse/single). The 3D heat conduction differential equations of central-substrate are given based on the above packaging structure. According to the solutions of numerical temperature field distribution, which are solved through a mathematic-modeling tool, the processing technique of central-substrate is optimized. And the prototype of multi-wavelength laser diode is prepared, and the experimental apparatus is built which can be used to observe the phenomenon of spectral peak thermal-drifting. Experimental results show that only two spectral peaks are drifting slightly 13 nm. The drifting amount is within the half width range of their spectral peak. This phenomenon proves that the output spectra of multi-wavelength laser diode have a high precision and a well thermal stability.