A large mode area bandgap fiber is fully investigated,in which the guidin g system is formed by high-refractive index rods.These rods are arranged in a mesh grid of the square structure.Losses and mode field area of the straight fiber and bending fiber are calculated by applying the finite-element method.The inv estigation results show that the transmission of higher-order modes and fundamental modes can be supported by th is fiber simultaneously,owing to a wide bandgap.In addition,leakage losses of these modes are less than 10 －3 dB/m.The calculation results indicate that cladding modes with strong leakage losses are generated from cladd ing,when bending the fiber. Under the condition of a certain bending radius,cladding modes are coupled with higher-order modes in the core. In particular,when the bending radius is in the range of 15cm to 20 cm,the bend ing losses of fundamental modes are less than 0.01dB/m,whereas losses of higher-order modes are more than 1dB/m.As a result,the large mode area single-mode transmission can be realized by bending the fiber.At 1064nm,the mode field area of the straight fiber can reach 1319.62μm²,but the mode field area of bending fiber can be more than 975μm²,which confirms that the large mode area and low be nding loss can be achieved.