Usually, the high-order-mode fiber is a kind of large mode area fiber with multi-cladding. In order to obtain a large mode area to suppress the fiber nonlinear effects and avoid optical damage in high power fiber lasers and amplifiers, the structural parameters of a high-order-mode fiber need to meet certain conditions. The characteristic equation of high-order-mode fiber with multi-cladding and step-index distribution is derived with the theory of wave equation. According to this equation, the basis and method of choosing structural parameters for high-order-mode fiber are also described. And through numerical calculation, it analyzes the influence of the fiber cladding radius and refractive index on the core guided mode. The result shows that, under certain conditions, the cladding structure of the fiber does not affect the mode field distribution in the core and the mode field distribution in fiber cladding of certain high-order-mode can appear uniform distribution.