A large mode area core erbium doped K9-glass single-mode photonic crystal fiber is designed by getting the pump and signal intensity profiles from plane wave expansion method(PWEM)and solving propagation-rate equations. Its diameter is 20μm,limit ellipticity error is 94.7%,and limit offset of inner ventage is 0.1 μm. Then we analyze its amplifier characteristics. From numerical analysis when concentration of the erbium ions(N_t)is 1×10²⁶,we get the threshold of this fiber amplifier is 180 mW,and the optimal length is 0.26 m.