Abstract: Based on the platform of ABAQUS/Explicit, a 3D elastic plastic finite element model of high strength 21-6-9 stainless steel tube NC bending process was established, and its reliability was validated. Then, the influence laws of different clearances between tube and dies on wall thickness change and cross section distortion were obtained by the model. The results show that increasing tube-mandrel clearance, tube-wiper die clearance or decreasing tube-pressure die clearance, tube-bending die clearance can reduce wall thinning ratio; reducing tube-mandrel clearance or increasing tube-pressure die clearance can decrease wall thickening ratio, while other clearances between tube and dies hardly have an effect on wall thickening ratio; reducing tube-pressure die clearance, tube-bending die clearance or increasing tube-wiper die clearance can make the cross section distortion of tube weaken, while increasing tube-mandrel clearance, the cross section distortion firstly reduces then increases. And the optimal clearances values among dies and tube are obtained for Φ15.88 mm×t0.84 mm×R47.64 mm high strength 21-6-9 stainless steel bent tube. The results may provide a theoretical basis for parameters selection and die design during tube NC bending precision forming process.