A method for designing and preparing the rectangular-wave broadband pass filter was put forward after a deep study. A rectangular-wave OD3-A broadband pass filter was designed and prepared with this method, with its center wavelength λ₀=515 nm in the working range of 400 nm~1 100 nm. And the mean transmittance of the sample in its pass-band of λ₀±25 nm was great than or equal to 92.7%, meanwhile the transmittance in the cut-off band λ ranging from 400 nm~475 nm and λ ranging from 555 nm~1 100 nm was less than 0.1%. The transmittance of the thin film sample was tested, and the spectrum met the requirements. The rectangular-wave broadband pass filter designed and prepared by this method overcame the disadvantages of high precision requirements of Fabry-Perot interferometer narrow-band filter film monitoring, narrow pass-band broadband, high cost, overlarge total thickness, low pass-band transmittance and poor waveform rectangularity in the traditional combination way of long and short wave cut-off films.