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; 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 according to the result of the adjustment procedure, the second vector group including the capacity vectors satisfying 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.