In Chemical Oxygen-Iodine Laser (COIL) system, the water vapor produced from the generator of singlet oxygen can have severe negative effects on the output of the laser power. In this paper, the water vapor content in the counter-flow Jet type Singlet Oxygen Generator (JSOG) is simulated. The results agree well with those of experiments. By way of simulation, some of the experimental conditions for JSOG are optimized to reduce the water vapor content.