For further investigating the fiber laser mode control, the bending properties of large mode area fibers need to be studied. Begining with the Helmholtz equation, by comparing the field distributions of straight fiber and bent fiber modes, the mode evolution ways between straight and curved large mode area fibers transition are determined. Based on mode superposition and excitation theory, the mode coupling efficiency formula about straight and bent fiber transition is derived. With the help of coupling formula and finite element method, the mode coupling efficiencies of a step-index large mode area fiber in straight and curved state transition are calculated. The results show that self-coupling and cross-coupling processes exist when fiber transits between straight and curved states, and the coupling degree varies with the degree of bending and mode order, in which the coupling efficiency of the fundamental mode is always higher than that of other modes and the coupling loss of higher order mode is larger than that of lower order mode. The fiber transition between straight and curved states costs the total power to exchange for the effective inhibition of higher order mode.