Mountainous rivers have the characteristics of rapid flow and large variation of flow & water level，which often lead to instability，displacement，and drift damage of the beacon vessel，becoming a threat to the beacon vessel. Selecting the typical accident section of the upper reaches of the Yangtze River，we carry out the experimental study on the instability mechanism of the beacon vessel and anchor stone system from the aspects of the water-resistance area of anchor stone，the weight of anchor stone，water depth，velocity，and cable angle. The results show that from the perspective of the anchor stone’s shape，the flat anchor stone has the advantages of low gravity center，large bottom area，and small water-resistance area，and has better stability and ultimate tension than the square anchor stone of the same weight. From water flow and ropes，with the increase of Froude number and cable angle，the stability of the beacon vessel and anchor stone system is worse. The smaller cable angle will lead to better stability. The empirical formulae between the ultimate tension，water depth，velocity，and cable angle are presented，which provide theoretical support for the layout of the river beacon vessel and anchor stone system of mountainous rives.