Objective　To get the length parameters in vivo of ligaments of healthy knees of Chinese during weight-bearing flexion after motion analysis with the 2D/3D registration technique.　Methods　2 orthogonal X ray images of knees were got under weight-bearing flexion 0°, 15°, 30°, 60°, and 90° in 8 healthy knees of volunteers. These orthogonal images were then imported into virtual fluoroscopic registration system and registered with the three-dimensional models which were reconstructed from CT data with the 2D/3D image registration technology. After 2D/3D image registration, the three-dimensional motions of knees under weight-bearing were showed. Through the bone insertion of ACL/PCL, MCL/LCL, the length parameters of ligaments in vivo were analyzed.　Results　With the flexion angle increased, ACL length gradually shortened, from （27.73±1.34）mm at 0° to （23.34±1.48）mm at 90°s; PCL length increased gradually, from （26.5±1.65） mm at 0° to （33.80±1.97） mm at 90°; MCL length decreased gradually, From（38.17±1.40） mm at 0° to （35.98±1.50） mm at 90°; LCL length gradually reduced, from （52.56±1.64）mm at 0° to （50.22±1.41） mm at 90 °. Differences have statistical significance (P<0.05).　Conclusions　The in vivo ligaments parameters of the healthy knee at different flexion angle can be got through the 2D/3D image registration technique.