We present a packaging structure of multi-wavelength infrared laser diode and study the beams collimation techniques under this structure for improving beams parallelism of wavelengths at 860 nm, 905 nm and 1064 nm (pulse/single). By the imaging analysis of Gaussian beam in both paraxial and off-axial conditions, we summarize and present an imaging theory of “quasi-coaxial” laser beams. The collimating optical system is designed by this imaging theory, and corresponding optical paths are modeling by the optical-design software. A prototype of multi-wavelength laser source is prepared under the guidance of optical paths model, and the parallelisms of “quasi-coaxial” beams are tested by a verification experiment. The experimental results show that the “quasi-coaxial” beams of 860 nm, 905 nm and 1064 nm (pulse/single) have well parallelisms.