A theoretical model is established for dealing with second-harmonic generation (SHG) in type I collagen excited by linearly polarized light focused by a microscope. With this model, the effects of the polarization angle , numerical aperture (NA), as well as the ratio of hyperpolarizability on SHG emission have been investigated. Simulation results reveal that SHG emission power changes periodically as . The use of lower NA leads to weaker SHG emission but is more concentrated in two closer lobes, whereas more distributed emission in two detached lobes appear at higher NA. As the introduction of polarization direction, which is not along with the fiber axis , one more element is valid in our case than alone, while their ratio plays a very important role for collagen features characterization. SHG emission with shows complicated modality that SHG emission is different at different and not symmetric at except at , suggesting the important impact of polarization working on for SHG emission. Our theoretical simulation results provide useful clues for experimental study of microscopic SHG emission in collagen excited by linearly polarized beam.