In order to explore whether radon anomaly could be considered the precursor of rock slide, the forming mechanism of the radon anomaly is qualitatively analysed according to the relationship between rock rupture and radon content. It is indicated that the rock microfracture may be one of the reasons leading to the radon anomaly before rock slide, suggesting that the CR-39 solid state nuclear track detector could be used to monitor the radon concentration in the region of the slide. The information may then be used to predict rock slide.