The parameters of a conventional power-system stabilizer（CPSS） are always identified offline without considering various operating conditions， therefore the CPSS cannot provide effective damping to restrain the low frequency oscillation of a power system. The paper presents an adaptive power system stabilizer based on the improved generalized predictive control method（GPC）. To avoid large amounts of computation of the traditional GPC， a fast algorithm is proposed which makes it not necessary to solve the inverse matrix. Moreover， the weighting coefficients in the objective function of the improved method are adjusted online by considering the influences of control increments and dynamic performances of a system. Simulation results show its satisfactory dynamic performances and regulation precisions in improving a system dynamic stability and damping low-frequency oscillations.