1. A system reliability evaluation method for transmission by a single minimal path in time restriction, using a computer containing an input unit, an operation unit and an output unit to execute a reliability evaluation software which provides a virtual network for simulating the flow network, the virtual network having a start node, a terminal node and plural arcs between nodes for constituting plural minimal paths, including the steps of: inputting a transmission requirement, a budget restriction and a time restriction from the input unit by users; distributing the transmission requirement in one of the minimal paths of the virtual network; according to the transmission requirement, the time restriction and a transmission time which is smaller than or equal to the time restriction, calculating a maximum flow of each arc of the minimal path per unit time accordingly, regarded as a capacity of the arc; defining a capacity vector composed by the capacities of the arcs, the capacities which are stochastic to correspond with the flow distribution state of the flow network; executing a budget check by the operation unit to check if a transmission cost satisfies the budget restriction for sending the transmission requirement in the minimal path; defining the capacity vector as a lower boundary vector when the transmission cost and the transmission time of the minimal path are less than or equal to the budget restriction and the time restriction respectively; calculating the probability that the capacity vector of any path is larger than or equal to the lower boundary vector of the minimal path, called a system reliability of the flow network; and displaying the system reliability on the output unit.