Quench ring is a key component of a coal water slurry gasifier. Hot process gas is quenched by contacting with black water in falling film formed on the inner wall of the vertical tube after it flows through the quench ring. A new rotary falling film quench ring for washing and cooling the hot process gas was developed without the shortcomings of the conventional quench ring. The velocity and thickness of rotary falling film in a vertical tube were measured using Dual partical dynamics analysis. The velocity of the falling film was affected by both distribution and flowrate of water influent, and the thickness tended to be steady at z=0.5 m. Relative to the conventional quench ring, the new ring produced more uniform falling water film on the inner tube.