Abstract: For the demand of narrow bandwidth and high finesse, we proposed a novel microloop filter with cross bus.Transfer function of drop port and through port were deduced based onthe signal flow graph theory and microring resonance theroy. The simulation and optimization were performed under the resonator wavelength of 1 550 nm, the results show that when the radius of the microring is 30.74 μm in the filter of signlemicroring with cross bus, the coupling cofficient between microringand channel waveguide is equal to 0.1, output spectrum from the through port has high finesse and this device belongs to periodic narrow-band high extinction filter.The bandwidth of output spectrum from the drop port is extremely narrow which can be used as a fixed wavelength laser. In the filter of microring resonator series-coupled with cross bus, the coupling cofficient between microringand channel wavegauide is equal to 0.05 and 0.000 4 between microrings, the output spectrum from the drop port has flat top and the rolloff in transition is obvious.The characteristic of this filter can be infered by numerical calculation and was found that the finesse is equal to 105.41 and the quality factor is 4.189×10⁴. Analysis results show that the novel microring filter maintains the excellence performance of filtering and improves the finesse significantly.