To improve the utilization efficiency of bandwidth in an interleaver system and satisfy the requirements of different transmission speeds for different passbands, a novel design scheme of 50 GHz unequal passband interleaving filter with an all-fiber MZI was put forward. Based on the principle of fiber transmissivity method, the MZI all-fiber optical different output spectra using three single-mode fiber couplers connected with single-mode fibers were analysed,and the complex amplitudes of output spectra were established with the expressions described by the coupling coefficients and length differences.Experiments indicate when the length differences of interference arms show the same value and the appropriate coupling coefficients of the couplers are chosen, the device can split an input beamwith channel space of 50 GHz into two beams,and one is odd channel for 10 Gb/s and the other is even channel for 40 Gb/s.Furthrmore,the space between the two channels is 0.8 nm and channel isolation is more than 25 dB.Finally,it points out that the interleaver is not sensitive to the coupling coefficients from three kinds of couplers and the coupling ratios of couplers can be controlled accurately.Obtained experimental results are in good agreement with the analytical ones.