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, a storage unit and an output unit electrically connected with each other, the computer executing 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: storing the reliability evaluation software in the storage unit; the input unit receiving a transmission requirement and a time restriction inputted by users, wherein the transmission requirement is data sent through the virtual network; defining a transmission time as a time for sending the transmission requirement from the start node to the terminal node, the transmission time being lower than or equal to the time restriction in the reliability evaluation software; defining each of the minimal paths as an ordered sequence of the arcs from the start node to the terminal node without loops in the virtual network of the reliability evaluation software; defining the transmission requirement as being divided into a first throughput and a second throughput, wherein the minimal paths of the virtual network include a first minimal path and a second minimal path predetermined in the reliability evaluation software, wherein the first throughput is variable and sent through the first minimal path, and the second throughput is variable and sent through the second minimal path; the operation unit calculating a plurality of combinations of the first throughput and the second throughput according to the sum of the first throughput and the second throughput equaling to the transmission requirement, and selecting a plurality of feasible solutions from the combinations according to the time for respectively sending the first throughput and the second throughput lower than or equal to the time restriction; the operation unit calculating a first minimal capacity of the first minimal path according to the first throughput and an operation relation that the time for sending the first throughput is lower than or equal to the time restriction, and the operation unit calculating a second minimal capacity of the second minimal path according to the second throughput and an operation relation that the time for sending the second throughput is lower than or equal to the time restriction; 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 the capacities of the arcs are defined as zero when the arcs of the first minimal path and the second minimal path are empty of the capacities, wherein each of the capacities is defined as a maximum throughput sent through one of the arcs in a unit of time; the operation unit 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, the operation unit 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.