Through analyzing the survivability theory of traditional information system, this paper puts forward an important index named DRUA (delivery rate under attack) to evaluate the MANET survivability, and an evaluation formula for this index is also provided based on the theory of space probability. In order to verify the formula, some experiments are conducted by using NS2 simulation tools. In these experiments, the famous reactive routing protocol AODV is selected as the communicating protocol. Other reactive routing protocols are designed similar as AODV, which has better performance. At the same time, three kinds of attacking scenes are designed for comparison (they are random attacking, deliberate attacking and randomly neighbor attacking). The experimental results show that when the number of CBR flows is comparatively small, random attacking and deliberate attacking have little negative effect on the performance of MANET or even can improve the performance of MANET. On the contrary, when there are a large number of CBR flows in the case of large-scale MANET, random attacking can greatly influence the performance of MANET in a negative way. This means the evaluation formula is more suitable for large-scale MANET. In addition, randomly attacking neighbor nodes can reduce the performance obviously even when there are a small number of CBR flows.