the present paper first introduced filter's filter principle as well as the analog filter, the digital filter's design method. Introduced the IIR digit filter's design method with emphasis. Namely pulse response not political reform and bilinearity method of transformation. Based on this, hypothesized realizes the random step IIR digit filter with DSP. This design extension is good, is advantageous adjusts filter's performance, may act according to the different request to perform on DSP to realize. key words: DSP filter, IIR digit low pass filter