Altered diabase is a kind of undesirable geological body often encountered in the construction of underground engineering in western China, which has a significant impact on the stability of the surrounding rock of tunnels. In this paper, based on the statistical analysis of the collapse of the tunnel face during the excavation process of the altered diabase diversion tunnel of the Dadu River Yingliangbao Hydropower Station, the main influencing factors of the stability of the altered diabase tunnel surrounding rock were analyzed by numerical simulation methods, and the matter-element extension theory was introduced to evaluate the collapse risk. The results show that the stability of the surrounding rock of the altered diabase tunnel is mainly controlled by the characteristics of the altered rock zone, the width of the alteration zone, and the condition of groundwater. On the premise of evaluating the degree of collapse by the height of collapse, the wider the alteration rock zone, the worse the characteristics, the richer the groundwater, the worse the stability of the surrounding rock, and the higher the risk of collapse in the cave roof in front of the working face. In addition, the outcropping position of the altered rock zone and the characteristics of the granite on both sides also have a certain impact on the stability of the surrounding rock. After the qualitative indexes were quantitatively processed, the width of the altered rock zone, the diabase alteration intensity index, the groundwater condition, the outcropping location and the surrounding rock granite alteration intensity index were selected as the evaluation indexes, and a collapse risk assessment method based on matter-element extension theory was established, and the collapse risk of the diabase alteration zone in the diversion tunnel was evaluated. The calculation grade is highly consistent with the actual situation, which provides theoretical support for the construction of water diversion tunnel and tunnel engineering.