Abstract： Objective To investigate the reasons uly patella baja occurred after primary total knee arthoplasty(TKA). Methods We retrospectively studied 380 primary TKA knees of 260 cases.Insall-salvati index was indicated to measure the height of patella and the variation of the joint line.We recognized such an clinical positive sign as the value of insall-salvati index ＞0.8 and proximally transfering joint line ＞4 mm.In mean term of follow up,subjects was divided to two groups according to HSS.The two groups took 75 score as a sorting baseline to analyze the correlation of patellar baja with the insall-salvati index and the proximally transfering joint line. Results There is no evdence of patellar baja in group of high scores of 352 knees.22 knees occurred patellar baja in group of low scores of 28 knees which joint line elevated(7 ± 2.0)mm.Patella baja were correlated with insall-salvati's index and elevation of joint line. Conclusions Patella baja occurred after TKA is multifactorial.We keep the original joint line maximally during TKA and stont knee rehabilitation as soon as possible after TKA.