For performing single mode (SM) single polarization (SP) operation in fiber lasers with flattened fundamental (FM) mode in large-mode-area, a method for obtaining SMSP signals with FM mode is presented. Two passive fiber cores with elliptical air holes are inserted into layers of fibers with FM mode. The cores are placed on both sides of an active core region with 20 μm diameter symmetrically and elliptical air holes are used to import birefringence. According to numerical simulation, when the active fiber is used as the gain medium of fiber lasers with FM mode, only polarization fundamental mode in x direction will be output by fiber lasers without choosing other modes.