Supercontinuum (SC) is a very promising technology which is widely used in a great deal of optoelectronics fields. In this letter, a SC producing process has been numerically simulated . The effects of chirp of the pump pulse on the width of SC are analyzed . and a series of conclusions are mentioned to explicate how optical pulse chirp affects SC when the SC fiber is DSF. Especially , the chirp effects using dispersion decreasing fiber (DDF) as SC fiber and the evolutions of SC along DDF of different chirp pulse are analyzed and compared . The threshold chirp for SC producing is defined. The simulation results are theoretical analyzed and some useful conclusions are given.