The linear photogalvanic effect (LPGE) is investigated by using the non-equilibrium Green's function (NEGF) technique combined with density functional theory (DFT) in monolayer Na2MgSn. We consider the cases of three different central regions, which are pure Na2MgSn, Na-vacancy, and Pb-substituted. It is found that both pure and defective Na2MgSn monolayers induce photoresponse under linearly polarized light. The photoresponse varies periodically as a form of either sinusoidal or cosinoidal function of the polarization angle. In the near-infrared and visible ranges, the photoresponse is more sensitive to the long wave range of visible light. In the case of single-atom defects, the photoresponse with Na-vacancy is larger than that of pb-substitution defects. Compared with the other two central regions, the maximum extinction ratio (ER) of Na-vacancy is larger, so it has higher polarization sensitivity. When the location of Na-vacancy is adjusted, the photoresponse changes obviously, and the Na 1*- vacancy has the largest photoresponse. With the increase of the Na-vacancy concentration, the photoresponse changes nonlinearly but is smaller than that of a single vacancy. A small bias voltage can greatly improve the photoresponse. Our results suggest an effective method to enhance the photoresponse and show the promise of Na2MgSn monolayers in optical detection.