By decreasing the radius of dielectric rods rather than removing a circle of dielectric rods in the photonic crystal ring resonator(PCRR), an annular line defect is constructed. A new type of photonic crystal ring resonator is proposed. By using the finite difference time domain (FDTD),the transmission property of lightwave through the ring resonator is analyzed. Further more, by changing the radius of dielectric rods, the influences on the filtering property is discussed. The result shows that the narrowband filter with single mode can be realized when the radius of rods is in the range of 0.43R~0.51R. At the same time, the center wavelength is close to 1550 nm at the third communication window, the rates of normalized transmission are all over 85% and the bandwidths are all below 17 nm. It provides available reference for the wavelength division multiplexing (WDM) multiplexer/demultiplexer.