Abstract: Considering the difficulty in fast analyzing and evaluating the radiator interaction between antennas which are in the same system, a fast analytical method for radiator interaction between antennas is proposed based on vector coefficient spherical harmonics. The calculation equation of coupling current between two antennas is derived by the electric field integral equation, and it is depended on the radiation pattern of the antennas. The spherical harmonics with vector coefficients are used to decompose the radiation pattern of the antennas. On one hand, several times of antenna modeling, meshing and EM analysis are avoided if the antennas are rotated. On the other hand, the problem of singular point induced by spherical harmonics with scalar coefficients is avoided. The analytical results are in good agreement with that achieved by some commercial electromagnetic software FEKO. The analytical time is reduced obviously, which confirms the correctness and availability of this method.