The invention relates to a light-weight precast concrete rib beam type stairway. According to the light-weight precast concrete rib beam type stairway, specific stairway structure construction and related built-in fitting reserving and arranging are included; the light-weight stairway belongs to a beam type stairway and can be designed as a downward-returning beam type stairway, an upward-returning beam type stairway and an upward-returning and downward-returning beam type stairway; a thin-wall folded plate type structure is adopted by steps of the light-weight stairway, specifically, step flat plates are supported on step vertical plates, and the step vertical plates are supported on rib beams on the two sides of the light-weight stairway; the two sides of platform sections at the two ends of the light-weight stairway are partially thickened to be reliably connected with the two ends of the rib beams, and the end parts of the platform sections are partially strengthened to enhance thestrength and the stiffness in the plane of the light-weight stairway; for the rib beams of the light-weight stairway, and when the upward-returning beam type stairway and the upward-returning and downward-returning beam type stairway are adopted, the platform sections of the light-weight stairway do not return upwards; and a folded beam type structure is adopted at the connection positions of therib beams of the light-weight stairway and the platform sections of the light-weight stairway.