Based on the wave equation, we directly analysis the guided modes in a helical fiber with circular cross-section and arbitrary profile of the refractive index in localco ordinate system. Two important results are obtained: (1) The guided modes in a helical fiber are right-rotated or left-rotated circularly polarised modes. (2) There don't exist TE and TM modes in a helical fiber. The theoretical results agree with that of the available experimental measurements.