We investigated the application effect of different concentrations of platelet-rich plasma (PRP) combined with quadriceps training on cartilage repair of knee osteoarthritis. Data of 37 patients with knee osteoarthritis (KOA) treated in our hospital (November 2019–February 2021) were retrospectively analyzed and the patients were divided into low concentration group (LCG) (n = 12), medium concentration group (MCG) (n = 12), and high concentration group (HCG) (n = 13) according to the order of admission. All patients received quadriceps training. Three groups above received knee injection of PRP, and the platelet concentrations were 1000–1400 × 109/L, 1400–1800 × 109/L, and 1800–2100 × 109/L, respectively. Articular cartilage thickness of the medial and lateral femur, knee joint function scores, inflammatory factor levels, and matrix metalloproteinases (MMPs) levels were compared. After treatment, compared with the MCG and HCG, articular cartilage thickness of the medial and lateral femur of the diseased side in the LCG was obviously lower ( P < 0.05 ). At 2 months after treatment (T3), compared with the HCG, articular cartilage thickness of the medial and lateral femur of the diseased side in the MCG was obviously higher ( P < 0.05 ), without remarkable difference in articular cartilage thickness of the medial and lateral femur of the healthy side among three groups ( P > 0.05 ). After treatment, compared with the LCG, knee joint function scores of the MCG and HCG were obviously better ( P < 0.001 ). Compared with the HCG, the knee function score at T3 in the MCG was obviously better ( P < 0.001 ). After treatment, compared with the LCG, inflammatory factor levels and levels of MMPs in the MCG and HCG were obviously lower ( P < 0.05 ). Compared with the HCG, inflammatory factor levels and levels of MMPs at T3 in the MCG were obviously lower ( P < 0.05 ). PRP combined with quadriceps training can accelerate cartilage repair of patients with KOA and reduce inflammatory factor levels and levels of MMPs, but the treatment effect of PRP depends on platelet concentration, with the best range of 1400–1800 × 109/L. Too high or too low platelet concentrations will affect recovery of knee function.