For autonomous landing of lunar probe, to develop autonomous landing and hazard avoidance technology is very necessary and the key of this technology is hazard detection. The focus of this paper was to present an algorithm for autonomous rock (one kind of hazard on moon surface) detection based on passive images. The paper could be divided into three parts. Firstly, Maximum 2D Entropy Thresholding was employed for detection of rock shadow. Secondly, the result of shadow detection was combined with rock contour extraction for more precise center point of ellipse which describes rock area. Finally, the parameters of this ellipse were computed and rock areas were confirmed. Experimental studies demonstrate that the algorithm of rock detection can describe scope of rock area more successfully than only shadow-based method.