Ecosystem risk is a new concept in understanding environmental problems. Studying and developing quantitative methods for regional ecosystem risk analysis is very important. In this study, some new indicators and methods for measuring oasis ecosystem risk are established using reliability theory. These indicators are linked to water resource, which is the key restricting factor in arid area oasis ecosystems. They have clear meanings and can also be compared in different arid area oases. A case study in the Liangzhou oasis of the Shiyang River Basin shows how to calculate these ecosystem risk indicators. The results of the case study are as follows: the reliability indicator, risk indicator, stability indicator, and integrated loss indicator of the Liangzhou oasis are 0.686, 0.334, 0.743, and 0.301, respectively. This means that the reliability degree of the oasis’s ecosystem safety is 68.3%; the degree of risk that it is unsafe is 33.4%; the stability degree is 74.3%; and 30.1% of the oasis’s area is supported by over-exploiting underground water and damaging the lower reaches of the ecosystem. This result can be used as a guide in controlling and managing ecosystem risk in the research area.