(MA)(2)Pb(SCN)(2)I-2, a new pseudohalogen-based 2D perovskite material, was reported as a very stable and promising photo-absorber in PSCs previously. However, the later researchers found that MA(2)Pb(SCN)(2)I-2 was not as stable as claimed. Thus, it is very critical to clarify the controversy and reveal the degradation mechanism of MA(2)Pb(SCN)(2)I-2. On the other hand, a large number of studies have indicated that adding a small amount of SCN- improves surface topography and crystallinity. However, whether SCN- ions can be incorporated into a 3D perovskite film remains debatable. In this work, the thermal degradation pathway of (MA)(2)Pb(SCN)(2)I-2 is revealed by thermal gravimetric and differential thermal analysis coupled with quadrupole mass spectrometry and density functional theory calculations. The decomposition of (MA)(2)Pb(SCN)(2)I-2 has been proved experimentally to be more complex than that of MAPbI(3), involving four stages and multi-reactions from room temperature to above 500 degrees C. By combining the experimental results and theoretical calculations, it is found that 2D (MA)(2)Pb(SCN)(2)I-2 actually is unstable when serving as photo-absorber in PSCs. Moreover, the role of SCN- in improving the crystallinity of 3D perovskite has also been discussed in detail.