The two methods measuring Einstein-Podolsky-Rosen (EPR) correlation of continuous variables, the balanced homodyne detection and the direct measurement for Bell-state are analytically discussed. In the two detection systems the designated reference levels for the quantum correlation measurement are different, so the displayed results on the spectral analyzer are not identical. After implementing a transformation to the reference level the measured squeezing of coupled mode, EPR correlation and the relation expression between them are totally same. The measurement principle of the two methods is theoretically analyzed and their characteristics are discussed from the requirements of experiments.