1. A network reliability evaluation method for an acyclic network, the acyclic network comprising a. plurality of nodes and a plurality of arcs connected to the plurality of nodes, the plurality of nodes forming a total node set V tot , the total node set V tot comprising a source node and a sink node, and the network reliability evaluation method comprising following steps: a. setting a node set V p comprising the source node and the sink node by a processor, setting a count value k to be 0, setting a minimal path set Ω to be a null set, setting a 0 th corresponding node set Θ 0 to be a null set, setting an upper limit value and a lower limit value corresponding to the acyclic network, and executing the step b; b. determining whether a number of the nodes in the node set V p is equal to the upper limit value by the processor, if so, executing the step f, if not, executing the step b′; b′. determining whether there is a first node that belongs to a set [V tot −(U i=0 k Θ i ∪V p )] and is an adjacent node of the node set V p by the processor, if so, the node set V p comprising the first node, adding the count value k by 1, setting the k th corresponding node set Θ k to be the first node, and executing the step c, if not, executing the step f; c. determining whether the lower limit value is greater than the number of the nodes in the node set V p by the processor, if so, executing the step b, if not, executing the step d; d. determining whether there is a path p formed of the plurality of arcs among the nodes in the node set V p passing all the nodes in the node set V p started from the source node to the sink node by the processor, if so, the minimal path set Ω comprising the path p and executing the step e, if not, executing the step d′; d′. determining whether there is a path p formed of the a plurality of arcs among the nodes in the node set V p without passing all the nodes in the node set V p started from the source node to the sink node or the other paths starting from the source node to the sink node is existing in the network formed of the set [V tot −(U i=0 k Θ i −V p )] by the processor, if so, executing the step b, if not, executing the step f; e. determining whether the number of the nodes of the node set V p is equal to the upper limit value by the processor, if so, executing the step f, if not, executing the step b; f. determining whether the number of the nodes of the node set V p is equal to 2, if so, executing the step g, if not, executing the step f′: f′. determining whether there is a second node that belongs to the set [V tot −(U i=0 k Θ i ∪V p )] and is the adjacent node of the node set V p , if so, the second node replacing the last node in the node set V p , the k th corresponding node set Θ k comprising the second node, and executing the step d, if not, deleting the last node in the node set V p , subtracting the count value k by 1, and executing the step f, and g. calculating a network reliability by the processor according to all the paths. comprised in the minimal path set Ω.