Root hemiparasitic Pedicularis kansuensis is an invasive native species in China and has been expanding in the Bayanbulak Grassland of Xinjiang Uygur Autonomous Region over the past decade, threatening the local livestock industry. To understand why this damaging species expands in some areas but not in others, we compared soil water content, soil nutrient status, and plant community structures between heavily infected and non-infected sites. We hypothesized that soil nutrient levels and plant species composition would be more beneficial to the spatial expansion of P. kansuensis in heavily infected areas than in non-infected ones. Quadrat (1 m × 1 m) surveys were carried out in the Bayanbulak Grassland. Species number, percent vegetation cover, aboveground plant biomass, plant height, and relative frequency were recorded. Three topsoil samples were taken at 0–10 cm depth from each quadrat and their nutrient statuses were determined. The results showed that (1) P. kansuensis invasion reduces the species richness of the community compared to the uninvaded area (3.07%), but increases the canopy cover overall (16.99%); (2) soil water content (SW) and soil nutrient content are the main factors that determine invasion by P. kansuensis, and SW plays the more important role; (3) SW controls P. kansuensis leaf stoichiometry, decreases the Ratio of leaf nitrogen (LN) to leaf phosphorus (LP), and affects LP (P < 0.005). Finally, after combining the results, we found that the soil factor accounted for 46.50% and plants accounted for 22.50% of P. kansuensis invasions in the Bayanbulak Grassland.