The invention relates to the technical field of automobile assemblies, and discloses a mounting structure of a built-in lower sliding-door guide rail. The mounting structure comprises a body-side outer plate, a step and a step supporting plate connected between the body-side outer plate and the step, wherein the step adopts a split structure and comprises a front step segment and a rear step segment; a lower-guide-rail mounting plate is also connected between the front step segment and the rear step segment; the lower sliding-door guide rail is fixed at the bottom of the lower-guide-rail mounting plate. Through the adoption of the mounting structure, the lower sliding-door guide rail is fixed under the step instead of on the side-body outer plate, so that the lower sliding-door guide rail is prevented from being positioned on two sides of a sealing surface of a sliding door; therefore, leakproofness at the sliding door is improved; besides, a layout space is saved, and the phenomenon that because a conventional sliding rail is exposed outside, the visual effect is influenced, is avoided; the working reliability of the lower slide rail is improved, and the service life of the lower slide rail is prolonged. In addition, the lower sliding-door guide rail is arranged under the step, so that improvement of the strength of the step is also facilitated, and the step is prevented from deforming.