A compound having the capability of releasing NO upon exposure to visible or near-infrared (vis or NIR) light could be a potential candidate for photodynamic therapy (PDT), which is significant for humans. Here, we investigated a series of Mn(II) complexes (a-d) based on density functional theory (DFT) to illuminate the mechanism of their behavior of releasing NO. Their structural, spectroscopic, and photodissociable properties were calculated by quantum theoretical methods to give a detailed and warranted explanation of the performance of releasing NO. The results indicate that, for a-d, releasing NO was attributed to the electron transfer from d(yz)/d(xz),(Mn) orbitals to pi*(NO) orbitals at the second excited triplet state (T-2). Importantly, we confirmed the finding in the experiment that d could release NO upon exposure to the NIR region and, thus, may be a best candidate for PDT in a-d. Therefore, to take d for example, the analyses of the potential energy curves (PECs) of difference states and electron density difference between the T-2 and the ground state (S-0) were performed to further provide evidence of ligand dissociation and release of NO at the T-2 state. Finally, we hope that our discussion can provide assistance to understand the behavior of the release of NO and design novel photodissociable transition metal nitrosyls for PDT applications.