We present an optical finite impulse response (FIR) filter design method. Based on crystal birefringence, an optical FIR filter can be designed to produce arbitrary spectrum output. A mathematic model of the optical FIR filter is introduced from the FIR theory. Details of the mapping relationship between an FIR filter and an optical filter are analyzed. A classical FIR filter design method is directly applied. After using the optical backward transfer method to implement the optical configuration, a design of a green/magenta broadband filter is discussed. The experiment results demonstrate that the theoretical simulation is in accordance with the experiment.