Phase-shift control can effectively avoid soliton interactions. With symbolic computation and Hirota's bilinear method, analytic studies are made on nonlinear Schrdinger equation, which can be used to describe the propagation of solitons in dispersion decreasing fibers. Analytic two-soliton solutions are obtained. With the obtained solutions, when the variable group-velocity dispersion function of dispersion decreasing fibers is a Gaussian one, the phase-shift control is achieved, soliton interactions are avoided, and the pulse quality in optical communication systems can be improved. Moreover, influences of parameters in dispersion decreasing fibers on the phase-shift control are discussed. Results are also helpful for the logic gates and optical switches.