Although nitride phosphors have been wildly researched, CaSiN2 is seldom reported. In this paper, two different phases CaSiN2, orthorhombic and cubic phase prepared by plasma-activated sintering were reported. Without active ions doped, the samples were pure orthorhombic phase. However, when Eu ions were doped into CaSiN2, the samples were consisted of both orthorhombic phase and cubic phase, and the ratio of two phases would change with the sintering temperature changed. Meanwhile, the photoluminescence (PL) intensity of samples would change adversely with the content of cubic phase. The differences between the structures of two phases and the existence form of Eu ions in different phases were reported. It can be inferred that the Eu2+ ions prefer to enter into cubic phase when two phases coexist, and the Eu2+ will be translated to Eu3+ easily due to the loose coordination environment in cubic phase, which will result in weaker PL intensity and worse thermal stability.