Abstract: In this paper, the space-charge weight functions of 1D and 2D finite-size macro particles for cylindrical system are derived using Green's function. The difference between space-charge field weight functions using the model of finite-size macro particles and those using the model of disk or ring1 is compared, and the more reasonable computing results are acquired. The output power of beam-wave interaction in helix TWT has been computed and compared with these two kinds of functions. The calculated results show that the output power of the finite-size macro particle model saturated sooner with a higher value than the other model.