1. An accurate method to evaluate a system reliability of a cloud computer network, executed by a computer providing a network model, wherein the computer is connected with a storage media and an operating unit through the cloud computing network, the network model comprises a plurality of arcs, each of the arcs has a current capacity, and the current capacities are arranged to form a plurality of capacity vectors corresponding to different states of the cloud computer network, the accurate method comprising the steps of: providing a total demand, a time constraint and a maintenance budget into the network model and storing the total demand, the time constraint and the maintenance budget in the storage media; the operating unit defining a first vector group, including the capacity vectors satisfying the total demand and the time constraint; judging whether or not the capacity vectors of the first vector group satisfy the maintenance budget; the operating unit executing an adjustment procedure to change the capacity vectors unsatisfying into satisfying the maintenance budget in the first vector group; defining a second vector group including the changed capacity vectors and the original capacity vectors satisfying the maintenance budget in the first vector group, so the capacity vectors of the second vector group satisfy the total demand, the time constraint and the maintenance budget; the operating unit calculating a probability according to the second vector group and defining the probability as an accurate value of the system reliability; and displaying the accurate value on an spreadsheet outputting from the computer.