For the purpose of improvement on operation economy and flexibility of the third-generation pressurized water reactor nuclear power plant, the stretch-out operation would be employed, which required flexible operation after the planned most economical cycle length. Usually, it happens at the end of cycle when the boron concentration is too low to induce reactivity, so as to the Reactor Coolant System (RCS) temperature coastdown is introduced, which leads to increase the reactor coolant fluid density and break flow, as well as reduce the pressure of containment, both of them have negative impacts on the results of Large Break Loss of Coolant Accident (LB-LOCA). And in the process, the fuel axial power deviation and radial power deviation have influence on the results. As a key accident of the design basis conditions, the analysis on the LB-LOCA under the stretch-out operation is necessary, which presented in this paper, and the analysis prove the results are satisfactory as well as the feasibility of stretch-out operation on the third-generation Pressurized Water Reactor nuclear power plant.