1. A system reliability evaluation method for transmission by two minimal paths in time restriction, using a computer including an input unit, an operation unit and an output unit to execute a reliability evaluation software which provides a virtual network for simulating a flow network, the virtual network having a start node, a terminal node and plural arcs between the start node and the terminal node for constituting plural minimal paths, the method comprising the steps of: inputting a transmission requirement and a time restriction from the input unit by users; distributing the transmission requirement in a first minimal path and a second minimal path of the minimal paths of the virtual network, and the first minimal path having a first throughput and the second minimal path having a second throughput; according to the time restriction, selecting a plurality of feasible solutions for satisfying the sum of the first throughput and the second throughput equaling to the transmission requirement; calculating a first minimal capacity of the first minimal path and a second minimal capacity of the second minimal path in the feasible solutions; defining a capacity vector composed by the capacities of the arcs, the capacities, which are corresponded with the flow distribution state of the first minimal capacity and the second minimal capacity, and which are defined as zero when the arcs of the first minimal path and the second minimal path are empty of the capacities; comparing the magnitudes of all the capacity vectors for defining the minimum capacity vector as a lower boundary vector; according to the lower boundary vectors, calculating a system reliability of showing all the capacity vectors satisfying the transmission requirement and the time restriction by using the set theory; and displaying the system reliability on the output unit.