Highly efficient yellow–green-emitting SrSi2O2N2:0.02Eu2+ phosphor with a well-dispersed and uniform morphology was synthesized by a direct silicon nitridation method. The related synthesis parameters including soaking temperature, holding time, silicon source, gas species, and flow rate, have been studied, respectively, in terms of phase compositions and luminescent properties. The possible reaction mechanism was also proposed based on the results obtained. Silicon is assumed to be transported to the surface of decomposed SrO particles in the form of SiO or Si vapor, and nitridation reaction occurs here to form SrSi2O2N2:0.02Eu2+ phosphor directly. The particle size and morphology of phosphors obtained are controlled by the raw oxide materials instead of silicon. CO2 from the decomposition of SrCO3 plays an important role in the transportation of Si, which reacts with Si to form SiO vapor and introduces excessive oxygen into the phosphor obtained, resulting in an O/N ratio close to SrSi2O2N2. Direct silicon nitridation is proved to be a simple, cost-effective, facile, and potential morphology-controlling method to synthesize (oxo)nitridosilicate phosphors of excellent properties.