WAMS(wide-area measurement system) provides a new method to improve the small disturbance stability of large-scale interconnected power grids. This article builds the unified mathematical model of time-delay and data-loss which are ubiquitous in WAMS through their mathematical expectations, thus obtains the quantitative result of data-loss to small disturbance stability. Based on the model, combining theories of delay-dependent state-feedback robust control and state observer, the additional damping controller is designed which can adequately consider the stochasticity of time -delay. Simulations are carried out in 2A4M system and a big power grid with different time-delays, data-loss ratios and models. The results demonstrate the correctness the proposed model. The damping controller designed can take the comprehensive characteristics of time-delay and data-loss into account and has a strong robustness, which have a prominent significance on the inhibition of low frequency and application of WAMS to large-scale power systems.