The invention discloses a spray-seeding medium for soil-rock mixed side slopes and a preparation method of the spray-seeding medium. The spray-seeding medium for the soil-rock mixed side slopes is mainly prepared from, by mass, 23-40% of clay, 50-75% of loamy soil and 0.6-12% of reed fibers, wherein the length of the reed fibers ranges from 20mm to 120mm. The spray-seeding medium for the soil-rockmixed side slopes are mainly prepared from the clay, the loamy soil and the reed fibers, which are compounded according to specific proportions; the structural stability, pulling force and adhesive force of the medium are enhanced through the reed fibers, and the degraded reed fibers serving as bio-organic fertilizer provide nutrients for plant growth; the problems caused after spray seeding of conventional media, such as poor structure, few granular structures and nutrient shortage of the soil, as well as the problem of unstability caused by efficacy loss of binders, are solved; resource utilization of reeds can be realized.