【Abstract】 Objective To explore the meaning of Platelet count to Mean Platelet Volume ratio and plasma Fibrinogen in the diagnosis of Prosthetic Joint Infection. Methods In a retrospective study, 110 patients presented with osteoarthritis, PJI, aseptic loosening after joint arthroplasy were divided into three groups according to the type of operation they received. 39 patients treated with primary arthroplasty; 42 PJI patients treated with resection arthroplasty and spacer insertion surgery; 29 aseptic loosening patients treated with revision arthroplasty. Record gender, age, serum CRP, ESR, Platelet count to Mean Platelet Volume ratio and plasma Fibrinogen level preoperatively in patients from the three different groups. Results There are no statistically significant differences when compared general data such as gender and age in patients from the three different groups. Serum CRP, ESR, PC/MPV and plasma FIB level in treated with resection arthroplasty and spacer insertion surgery are much higher than patients treated with primary arthroplasty and aseptic loosening patients treated with revision arthroplasty. When PC/MPV>31.70 was set as the optimal threshold value for the diagnosis of PJI, the specificity of PC/MPV in diagnosis of PJI was much lower than CRP and ESR, whereas, the sensitivity of PC/MPV in diagnosis of PJI was similar with CRP and ESR. When plasma FIB> 4.01 μg/mL was set as the optimal threshold value for the diagnosis of PJI, the specificity and sensitivity of plasma FIB in diagnosis of PJI was similar with CRP and ESR. Conclusion PC/MPV should not be selected as the first option for PJI diagnosis, whereas, the plasma FIB can be used for as a new marker for PJI diagnosis.