Considering the difficulty in the fabrication of dispersion decreasing fiber(DDF)in dispersion management systems,a new dispersion management line is designed to substitute DDF. By employment of two types of fiber with positive and negative group-velocity dispersion(GVD),and simple variation of the lengths of the normal and anomalous fiber sections,the average dispersion decreases from one dispersion map to another within the amplifier span follow the decrease of the energy,which can balance the local nonlinearity. The results of numerical simulation indicate that the ladder curve approximation of DDF's GVD can substantially improve the performance of the 40 Gbit/s optical transmission system.