Eco-security as a hot research area in Resource and Environment Science, not only assessments secure sate, but also relates to potential danger factors. It is a function of secure state, potential danger, evolving trend, time, space and object. Only integrating secure state with danger, secure trend can be judged accurately, and early warning can be realized. A case study of Liaohe River has developed a method of eco-security warning in this paper. The method includes 4 steps: ①Establishing eco-security state—danger evaluation indicator system. ②Evaluating secure state index by fuzzy membership degree. ③Assessing potential danger index by accident analyzing method of Graham. ④Calculating early warning index by multi-objective decision-making rule. The results are consistent with autoptical work. The method is verified that we can better know the status of warning.