In this paper, based on a variable-coefficient nonlinear Schrodinger (vcNLS) equation, amplification of the fundamental and second-order unchirped solitons in the dispersion-decreasing fiber without any external amplification device, which is different from those in the existing literatures, is studied. Via symbolic computation, soliton solutions of the vcNLS equation are obtained. For a fundamental-soliton pulse, the amplitude is amplified by the gain during the propagation, whereas the width keeps unchanged. Because of the equilibrium between the gain, nonlinearity and varying dispersion, soliton structure is not destroyed, and the amplified fundamental soliton is free from the pedestal and chirp. With the increase of the absolute value of the gain coefficient , magnification of the fundamental-soliton amplitude is enhanced in the same propagation distance. For the second-order soliton, the width is compressed and the amplitude is amplified, because the amplification process is accompanied by the compression of the soliton. Period of the second-order soliton decreases exponentially during the propagation, and decreases with the increase of the absolute value of in the same propagation distance.