In order to realize the optical multi-wavelength selector which is capable of selecting multiple wavelengths with unequal wavelength interval as needed in the free spectral range (FSR) νFSR, the algorithm of finite impulse response (FIR) digital filter is transplanted to the optical filter with the Mach-Zehnder interferometer (MZI) cascaded coupler structure. The target transfer function matrix is represented by discrete Fourier transform (DFT). Using method of undetermined coefficients, the array concrete physical parameters (kth-order coupling coefficient angle θk and kth-order phase shift value of φk) which the transfer function desires are derived from the transfer matrix. This kind of multi-wavelength selector is capable of selecting single or multiple wavelength of light wave randomly as needed in FSR, and the wavelength interval is unequal. Analyzing all the array parameters and simulation figures, the selector realizes nearly 0 dB transmittance on the point of the desired wavelength and the isolation is up to 12 dB. The 3 dB bandwidth of each wavelength and minimum wavelength interval decrease with increasing the cascading order N. When νFSR 1000 GHz and N=15, the 3 dB bandwidth of each wavelength achieves 0.5 nm and the minimum wavelength interval reaches 1 nm.