1. A shading compensation method of an image capturing device, adapted to an image capturing device having a primary lens, a secondary lens, and a diffuser, wherein the image capturing device pre-stores a primary-secondary lenses table associated with the primary lens and the secondary lens as well as a secondary lens-diffuser table associated with the secondary lens and the diffuser, and wherein the shading compensation method comprises: covering the secondary lens by using the diffuser; capturing a scene simultaneously by using the primary lens and the secondary lens covered by the diffuser so as to respectively obtain a primary image and a diffused image; constructing a shading compensation table corresponding to the scene by using the primary-secondary lenses table and the secondary lens-diffuser table based on the primary image and the diffused image; and performing a shading compensation process on the primary image according to the shading compensation table so as to generate a compensated image.