An evaluating method for internal network performance by simulation of a flow network model with an acceptable threshold for the required flow of the internal network is applied to a computer system. The network model comprises plural nodes and document flow paths, wherein each said node corresponds to one employee of the internal network with a capacity, which is tested to get a testing result, and each said document flow path has a flow, which maps into a probability distribution data by a probability mapping function. According to the probability mapping function and operation relationship between the capacity and the flow together with the acceptable threshold and the probability distribution data, a system reliability value is calculated out and shown in a display screen.