Considering load uncertainty， an interval analysis method of small-signal stability is presented when the load is expressed as an interval number. Based on the eigen-sensitivity of augmented matrix， the sensitivity of the eigenvalue damping ratio with respect to the active load power is derived. By using the guide of the sensitivity， the damping ratio extreme values of weak oscillation modes are solved by iterative search method under typical load levels， and the damping ratio distribution in the whole load range can be further determined. Accordingly the characteristics of damping ratio changes with respect to load variations can be obtained， so that the more comprehensive stability information can be provided. Finally， the validity and applicability of this method are proven through a 4-machine system and the New England 39-bus system. Comparison results illustrate that the method proposed is correct and superior to the Monte Carlo simulation method on the calculation speed.