A triangle-platform-index core fiber is proposed, in which the refractive index distribution in the cross-section of the fiber core is composed of several closely connected triangles and a platform at the outermost layer. The simulation results show that the proposed fiber possesses an extremely large mode field area of 3110 mu m(2) at the wavelength of 1.06 mu m, and achieves single-mode operation state near a bend radius of 17 cm. Furthermore, several different mode field distributions, including Gauss-like distribution, flatten distribution and hollow distribution, are achieved. The proposed fiber can benefit suppressing nonlinear phenomenon to increase the output power level of fiber laser, and promoting special industrial processing.