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; defining the transmission requirement sent by a single one of the minimal paths of the virtual network; defining a transmission time for the transmission requirement to be sent from the start node to the terminal node in the virtual network, wherein the transmission time is smaller than or equal to the time restriction; according to the transmission requirement, the time restriction and the transmission time, calculating a maximum flow for each of the arcs of the minimal path per unit time, and defining the maximum flow as a capacity for each of the arcs; defining a capacity vector composed by the capacity of each of the arcs, wherein the capacity is stochastic to correspond with the flow distribution state of the flow network; defining a transmission cost for the transmission requirement to be sent from the start node to the terminal node in the virtual network; executing a budget check by the operation unit to check if the transmission cost satisfies the budget restriction; 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.