A large mode area fiber with step index core and inner microstructure cladding is proposed. By using step index core and inner microstructure cladding formed by high doped rods with a ring array, the contradictory constraints of large mode field area and single mode in traditional structure can be effectively solved. Furthermore, high bend loss and sensitivity to bend orientation are also eliminated. The optimization of fiber properties are carried out by using full-vector finite element model with perfectly matched layer. Research results show that large mode area reaches up to 1 412 μm2 and the loss ratio of high-order mode to fundamental mode is up to 767 at the wavelength of 2 μm when the fiber is bent at the bend radius of 10 cm. In addition, fiber properties are independent of bend orientation. Due to its advantage of large mode area, excellent single mode, low bend loss and insensitivity to bend orientation, the proposed fiber is of great significance to promote the development of high power miniaturized fiber lasers.