The paper has 3D nonlinear analysis for TWTs. Based on macroparticle model, the electron beam can be subdivided into 3D Macroparticles to calculate space charge forces using Green's function methods, and 3D large signal working equations are obtained. The numerical results for a uniform magnetic focusing field indicates that, in 3D numerical analysis,3D spacecharge forces can be substituted by 2D's with little influence on the numerical results, which greatly decreases computing hours so that 3D computer program can be easily used．