<p>Nestled in the north-central expanse of the Qaidam Basin, the Mahai Salt Lake situated within the Saishten Depression boasts a profusion of low-grade solid potash ores of considerable economic significance. This study employs various test methods, including rock thin-section identification, X-ray powder diffraction, and scanning electron microscope–energy-dispersive X-ray spectroscopy, to analyze the mineral composition and structure, and investigates the depositional characteristics and genesis of polyhalite in low-grade solid potash ores. The results show that polyhalite is mainly distributed within the easily soluble layer, situated between halite crystals and detritus, with higher mineralization. Primary and secondary polyhalite genesis are discerned. Primary genesis by protogenic manifestation arises through the infusion of Ca-rich chloride brine into K–Mg-rich inter-crystalline magnesium sulfate subtype brine, within the convoluted evolution of multi-phase neotectonic movements, hydrological system adjustments, and recurrent arid-cold climate. Secondary genesis occurs by the metasomatism of gypsum and bassanite. The thickness and content of polyhalite in different layers display a linear trend from the northwest toward the east-central direction. This distribution is primarily governed by regional tectonic activities, establishing the east-central area as the focal region for polyhalite enrichment. The shallow distribution of polyhalite necessitates the need for the development of a suitable scheme for water-level control to optimize mining efficiency.</p>