The system difference between the link of cascaded line-amplifiers and the link of cascaded optical add-drop-module(OADM) / optical cross-connection(OXC) nodes is pointed out. A new formula to estimate the optical signal-to-noise ratio(OSNR) for the link with cascaded OADM/ OXC nodes in all-optical network(AON) is presented. Based on the formula, a set of OSNR results is given. The analysis and numerical result show that the OSNRformula for link with cascaded line-amplifiers, which is widely used in engineering designing, could not be applied directly in AON design without correction.