The site condition was complex, where the Second Tailrace Surge Chamber for the Jinping First Stage Hydropower Station was built . During the excavation of JPTSC2 between the chainage Nos. 0＋320 to 0＋360 degrees and from 0＋0 to 0＋120 degrees, falling of rock blocks often occurred. They were induced by the complex geological conditions, which was a big issue. So it was very important to judge and forecast where the big block would fall. After comprehensive analyzed, three conditions causing the big block falling were put forward. The first condition was that the rock block had to be cut by joint Nos. ①, ④ and the fault f18. The second was that the scale of rock block would be decided by the extended length of joint Nos ① and ④. The third one was that the block should be found at the lower side of the fault f18. Then searching work was carried on the three conditions. Finally a big block was found, its volume would be about 50 m3, its weight would be about 133.5 tons. Block Theory was used to calculate,the safety factor of the block of 1.05. It would be dangerous. So, some relevant countermeasures are suggested.