It seemed to be common sense that malnutrition was associated with osteoporosis, but there were few studies with detailed data proving that. Additionally, the association between BMI and osteoporosis was still under discussion. In our study of 138 patients, we first confirmed the association between nutrition and osteoporosis with propensity score matching method reducing the confounding bias, then discovered that body mass index (BMI) and 25OHD level acted as two crucial factors of nutrition risk-mediated osteoporosis. Moreover, a new BMI classification was proposed in our article to do more help for nutrition management and anti-osteoporosis treatment for the old in China. Consequently, nutrition is important to bone health, with BMI and 25OHD level playing key roles.