Plateau pikas (Ochotona curzoniae) play a keystone role in the alpine meadow ecosystem on the Qinghai–Tibetan Plateau (QTP). For decades, alpine meadows on the QTP have suffered from severe degradation, predominantly due to livestock grazing and frequent dramatic increases in plateau pikas. The cause has long been thought to be that livestock grazing provides open habitats for pikas. However, open habitats with low food nutrient quality are insufficient to maintain high-density populations. Feeding facilitation is a process or action undertaken by one species whose activity benefits another species. However, the facilitative effects on population consequences remain unclear. Here, we examined the impact of feeding facilitation on the population dynamics of plateau pikas. We hypothesize that the high protein amount provided by meadows that have been overgrazed would be an important factor leading to dramatic increases in plateau pikas. We found that plateau pikas preferred high-protein foods, and relatively abundant high-protein food resources were found in meadows following high levels of livestock grazing. Body mass and amount of protein in stomach digesta of pikas were higher in moderately and heavily grazed meadows than in lightly grazed meadow. Thus, pika population density was associated with high-protein food resources. A supplemental protein experiment confirmed that high-protein foods led to higher pika population density by increasing reproductive performance. Livestock grazing increased the abundance of plateau pikas by shifting plant nutrient contents toward higher protein conditions favorable to them. Feeding facilitation is an important factor that causes a dramatic increase in plateau pikas.