For studying the EM signatures of full-size target, it demands outdoor or indoor room large enough for measurement, but the construction of outdoor and indoor room which satisfy the far field conditional both lead to costly installation and require extraordinary elaborate technique, say nothing of carrying out the imaging and RCS diagnostic for stealth weapon in active service. This paper meets the demand of the development with the stealth and anti-stealth weapon, aims at the shortage of outdoor and indoor measuring of the far field electromagnetism feature, presents a research on 3-D near-field to far-field transformation algorithm, in which the far-field condition could not be satisfied on both vertical and horizontal plane for the target under test, and the target is illuminated by a spherical wave. A 3-D multi-scattering center model is built for the target and the near-field data is sampled by 3-D scan, using the extensions of spherical-wave function in cylindrical coordinates, the relationship between far-field and near- field scattering field is deduced under the condition that the antenna works at cylindrical-scan mode. The RCS and image pattern can be achieved by the 3-D IFFT algorithm and interpolation technique. The simulation results show that the algorithm is precise and effective.