The problem of small-signal stability in power system considering uncertainty of load and generation is investigated. Firstly， this paper builds a nonlinear optimization model for solving the upper and lower limits of oscillation mode's damping ratio under interval uncertainty. Then， an effective successive linear programming（SLP） method is proposed to solve this problem. By using this method， the interval damping ratio and the corresponding operation states at its interval limits can be obtained. Calculation results can be used to evaluate the influence of load and generation variations on a certain mode and give useful information for improvement. Finally， the proposed method is validated on two test systems.