Abstract: The emulsion explosive which is loaded by trucks in Yunfu sulfur-iron open-pit mine always self-ignite and self-blast when it is in borehole.In order to solve the problem, field contacting tests were developed between high sulfur mine and emulsion explosive. Taking the temperature of borehole and the result of tests as criterion of self-ignite fatalness directly, the reason and mechanism why emulsion explosive in borehole will self-ignite was analysed. A preventive measure was presented in this paper.