Polarization purity of antenna is an essential technical indicator in radar or communication system.However, the poor cross-polarization level caused by non-ideal orthogonality of dual-line polarized antenna makes the imperfect performance of system. In response of this problem, the quantitative relationships among the polarization angle of electromagnetic waves synthesized in space, the phase value and the phase error of orthogonal channel are given by theoretical deduction. Finally, the experimental results verifies the correctness of the analysis.