A nonlinear optical loop mirror (NOLM) constructed with linear dispersion decreasing fiber (DDF) is used to compress a beat signal. Several factors impacting on quality of output pulse in this compression system, such as dispersion slope of DDF, power-splitting ratio, incident pulse shape and peak power, are analyzed numerically. The new method for selecting device characteristics is adopted to enable both good pedestal suppression and pulse compression. As a result, the output pulse train with tunable and high repetition rate, pedestal energy of 5.09%, compression ratio of 25.6 and energy transmissivity of 50.56% is obtained by using 0.524 km-long DDF with dispersion slope of 26 ps²/km² and a coupler with power-splitting ratio of 0.54.