In order to improve the transmission performance of the all-fiber three-channel interleaver, a novel asymmetrical structure of a Mach-Zehnder interferometer(A-MZI) with three fiber arms is proposed. The A-MZI consists of a fused planar 3×3 coupler, a fused triangular 3×3 single mode fiber coupler and three fiber arms in series,in which the planar 3×3 coupler is used as an input coupler, and the triangular 3×3 coupler as an output coupler. The output expression is derived and the numerical simulation is performed.The simulation indicates that the crosstalk characteristics and the coupling-coefficient-angle tolerance can be improved greatly, and the sidelobe level of channel of the proposed device has been reduced more than 13.6 dB as compared with that of the conventional all-fiber three-channel interleaver. A interleaver is fabricated with the fused biconical taper technics in an experiment. The experimental results are in good agreement with the analytical ones. The result shows that the measured sidelobe level and the insertion loss are less than -34.6 dB and 1.0 dB,respectively, and the channel isolation is higher than 30 dB.