Abstract: For the convenient engineering application of complete quadratic combination(CQC) method in structure seismic response, it is simplified by the Seismic Code for Buildings. This paper discusses principally the parameter's change rule and the possible simplifying approach of mode correlation coefficient in mode combination formula, through compared and analysis for the two simplifying methods, then a new simplifying formula whose applied range is more extensive is proposed. The new formula adapts to the combination calculation of neighboring mode responses and its self-shake frequency is near, however its damping ratio has more difference.