The structures, energetics, spectroscopies, and stabilities of the singlet and triplet Si2NP isomeric species are explored at the DFT/B3LYP, QCISD, and CCSD(T) (single-point) levels. A total of 23 minimum isomers and 32 interconversion transition states are located. Generally, the triplet species lie energetically higher than the singlet ones. At the CCSD(T)/6-311+G(2df)HQCISD/6-311G(d) level, there are two nearly isoenergetic singlet isomers as global minima, i.e., a linear SiNSiP (1)1 (0.0) and a puckered cSiNSiP (1)4 with P-N cross-bonding (-0.6). The third low-lying isomer is a planar four-membered ring cSiNSiP (1)5 (3.3) with Si-Si cross-bonding. All the three isomers have reasonable kinetic stabilities and might be observable either in laboratory or in space. Moreover, the structural and bonding analyses indicate that (1)1 possesses typical SidropP triple bonding. Since up to now, stable SidropP triply bonded species are still experimentally unknown, isomer (1)1 may represent such a good example. Finally, the similarities and discrepancies of Si2NP are compared to the other analogous 18-valence electrons species AA'BB' (A, A' are group IV elements and B, B' are group V elements) such as C2N2 and Si2P2. The present study is the first one to consider such series with three higher-row atoms.