Firing temperature and gas pressure effect of synthesizing Sr2Si5N8:Eu2+ were investigated. The emission intensity is positively correlated with the firing temperature under 0.1 and 0.5MPa gas pressure. The Sr2Si5N8:Eu2+ with the highest emission intensity was found at 1700 degrees C and 1980 degrees C under 0.1 and 0.5MPa gas pressure, respectively. Although the maximum emission intensity of Sr2Si5N8:Eu2+ obtained under 0.5MPa gas pressure condition is higher than that under 0.1MPa. The Sr2Si5N8:Eu2+ synthesized under 0.5MPa gas pressure in the temperature range from 1600 degrees C to 1800 degrees C have lower emission intensities than that synthesized under 0.1MPa indicating that the melting of Sr3N2 is an important step for the formation of Sr2Si5N8:Eu2+. Moreover, the Sr2Si5N8:Eu2+ undergoes phase transition into SrSi6N8:Eu2+ completely after elongating the heating duration to 6h at 1980 degrees C under 0.5MPa gas pressure. The same feature was observed under 0.1MPa gas pressure after firing 8h at 1750 degrees C. Different heating durations led to different degrees of phase transition.