The invention discloses a cloud computing system reliability modeling method considering a common cause fault, and belongs to the technical field of network reliability. The method comprises the steps of determining a state combination of a similar single server of a cloud computing system and performing simplification; calculating an existence probability of the simplified state combination of the similar single server by adopting a fault tree method; determining state combinations of similar servers of the cloud computing system, performing simplification, and calculating an existence probability of each state combination; enumerating state combinations of different servers of the cloud computing system, and calculating an existence probability of each state combination; and according to the state space of the cloud computing system, calculating the system reliability according to a given demand. According to the method, a common cause fault of all virtual machines running in the servers, caused by server faults, is considered, the state space modeling is adopted, and the state space is simplified, so that the problem of state space explosion during system scale increment is solved and the modeling efficiency is improved.