Abstract: There are a lot of discontinuous planes in rock mass of Jinping II hydropower station conveyor tunnel,so it is of practical value to make certain whether there are key blocks on excavation planes where joints interlace with each other.In this paper,the stochastic probability model is drawn into key block theory to study the sliding probability of key block in Jinping II hydropower station conveyor tunnel on the basis of detailed geologic investigation.And some following conclusions are obtained.The relationship between trace length and the probability of instability of key block is inverse ratio.The probability of primary structural planes with 1～3m trace length is relatively higher.Key block containing structural plane J2 is relatively stable and the reinforcement of the arch would be crucial in the conveyor tunnel.These are all useful to offer effective reinforcement design and have important engineering values.