This study identifies the characteristics of a gas-centered liquid annular sheet in order to focus on the instabilities of the spray. To characterize the spray oscillation of the gas-centered liquid annular sheet, two measurement methods were used. Indirect photography was used to measure the instability wavelength and to distinguish the instability mode of the liquid sheet, and laser diagnostics were used to measure the frequency of the instability. In this study, three instability modes were observed, each mode had a different spray structure, and the instability characteristics of each were analyzed. As the gas-liquid momentum ratio increased, the wavelength of the spray oscillation decreased and the frequency increased. With an increase of the recess ratio, the wavelength increases and the frequency decreases. Consequently, it was clear that both the gas-liquid momentum ratio and the recess ratio can affect liquid sheet instability. The instability transition map can be obtained. The instability mode of the spray oscillation by normalized wavelength was identified.