Coherence light source with high brightness has important promising in the applications of X ray holography, three dimension imaging and X ray microscopy. On the basis of coherence phase volume analysis, the coherence mode, spectral brightness and photon flux etc. for undulator source in the third generation synchrotron radiation are illuminated in detail. It is important to the improvement of the efficiency for coherence energytransmission and of the soft X ray beam line design for X ray coherence experiments.