This paper proposes an improved phase parameter analysis and synthesis method for parametric stereo audio coding. For stereo signal with low interchannel coherence (IC), conventional phase parameter analysis and synthesis methods show poor quality, since they overlook the effect of IC to the phase parameters defined. The proposed system defines new phase parameters reflecting IC information, so that we can alleviate the deterioration of audio quality due to the synthesis of excessive phase difference, especially when IC is low. Subjective listening test results are presented to show that the proposed method provides always better audio quality than the conventional methods.