Many Ba shu bronze weapons are mottled. The corrosion of these weapons is different from those without mottle. In order to solve their problem of rust and attain effective preservation, it is necessary to study the corrosion mechanics. SEM-EDS, XRD and microscopy were used in the analysis. This paper is a preliminary discussion of the corrosion mechanics. By using SEM-EDS to analyze three samples, it is shown that the tin content is about 12%. The lead content is lower. The micro-structure of the mottle layer is different from that of the bronze body. The tin content of the mottle layer is also much higher than that of its bronze body. The thickness of the mottle layer is about 20-40[micro]m. The boundary between the matrix and the mottle layer can be seen clearly. SnO2 and Cu41Sn11 are the main substances of the mottle layer. The corrosion products of the non-mottle layer are oxidant of copper and some stable salts. The powdered corrosion of chloride is not severe. The different colors of the mottle layer and the matrix were caused by diverse corrosion products when they contacted the burial environment. SnO2 is the main product of the mottle layer. It forms a refractory surface to prevent further corrosion and protect subjacent matrix alloy. Non-mottle layer corrosion is extended to the matrix under the mottle and causes the mottle to drop off. The key to protect the weapons is to control non-mottle corrosion.