The green-emitting SrSi2O2N2:Eu2+ phosphors with single phase have been successfully synthesized by a simple two-step method. During the first-step process, Sr2SiO4:Eu3+ precursor was formed and emitted red light under UV light excitation. The excessive amount of Si3N4 in reactants promoted the formation of pure SrSi2O2N2 phase when molar ratio of Si3N4/Sr2SiO4 was located in the range of 1.1-1.5. The luminescence intensities of SrSi2O2N2:Eu2+ phosphors were enhanced by Si3N4/Sr2SiO4 molar ratio of 1.1. Phase analysis, luminescence properties and thermal stability of the samples were intensively investigated by X-ray diffraction, SEM, XPS and TGA/DSC. The trivalent europium ions were identified by XPS results on the surface of as-synthesized SrSi2O2N2:Eu2+ particles with approximate theoretical atomic ratio of Eu/Sr, which prevent the further oxidation of Eu2+ to Eu3+ in the bulk structure and evidently improved the thermal stability of the phosphors. For the thermal oxidation mechanism, it was suggested that the oxidation of SrSi2O2N2 to form SrSiO3 layer firstly occurred on the surface after a calcination process in air.