Disclosed is a modular stair structure for a fabricated building. The modular stair structure comprises: a lower floor beam; an upper floor beam; a stair landing, and stepping assemblies for connecting the stair landing to the lower floor beam and for connecting the stair landing to the upper floor beam, with the stair landing and the stepping assemblies being located between the lower floor beam and the upper floor beam; two main boards arranged on stepping assemblies in a face to face manner; a plurality of stepping plates arranged between the two main boards; and connecting members used for connecting the main boards and the stepping plates, wherein the plurality of stepping plates are sequentially and obliquely arranged downwards; the faces of the two main boards which face each other are the inner surface of the respective main board; the connecting members are respectively mounted on the inner surfaces of the two main boards; and each stepping plate comprises a horizontal panel and a vertical panel. The present invention achieves a fully fabricated design of house steps, prevents the steps from being assembled by means of welding on a construction site, simplifies the assembly process, improves the assembly speed, and achieves integrated, convenient mounting of house steps.