Prescombe Down is one of five National Nature Reserves in the chalk downland of south Wiltshire, part of the extensive plateau of Cranborne Chase, eight miles east of Shaftesbury. The site is not much visited since it is not signposted, and away from paved roads, requiring a walk of up to 1.2 miles to reach, and is also relatively small, comprising part of a broad, dry valley, 0.7 miles long, sloping southwards and divided at its upper end into two little forks.
The NNR is part of the larger Prescombe Down Site of Special Scientific Interest, which additionally contains a tributary valley to the east, Church Bottom. Like all combes hereabouts these have flat floors, fairly wide, enclosed by moderately steep slopes covered by short, calcareous grassland, while above, on top of the plateau, are regular farm fields.
As with the other NNRs nearby, Prescombe Down is so designated because of the rich variety of chalk grassland wildflowers, rarest being the early gentian which occurs quite abundantly across some of the south- and southwest-facing slopes. The reserve is also noted for insects, especially butterflies, such as the marsh fritillary, dark green fritillary, grizzled skipper, dingy skipper and adonis blue. The reserve is grazed by cows, who help reduce the growth of scrub and the more vigorous grasses, allowing more delicate plants to flourish. Orchids found in the valleys include autumn lady's-tresses, fragrant and pyramidal.
The nearest settlement to the reserve is Ebbesbourne Wake, situated in a larger valley to the south. A little way west of the village, along a minor road with little or no parking, a path, or right-of-way, heads northeast over two fields and across the lowest end of the NNR valley then up Church Bottom, and through another a field to a bridleway (Old Shaftesbury Drove), which 0.9 miles east meets a road (with parking space), between Fovant and Fifield Bavant. The bridleway runs alongside the north edge of the reserve and although thick vegetation prevents direct any access here, the upper end of the valley, the best area for wildflowers, can be accessed from the east by another path branching off the main walkway. The junction of the two forks in the valley is 1.3 miles via this latter route or 0.8 miles if walking up from below, from Ebbesbourne Wake. Wildflowers are fairly well spaced throughout the site, albeit with some variation due to the different aspects, and the consequent difference in moisture levels and amount of sunshine. The uppermost section of the valley, along the north edge of the reserve, is more overgrown, with bushes and trees, but the remainder is all grassland. Anthills are abundant in some locations.