We analyze the excess area and depth to detachment method developed by Epard and Groshong (1993), and apply it to the sand box model of Ge et al (2004) to illustrate that inadequate consideration will affect the calculation of true depth to detachment. Using the data of Yu et al (2006)to fit linear regression lines, we obtain the depths to detachment of Kela-2, Misikantage anticline and Dongqiu-8 structures, 115.74km, 14.17km, and 75.48km below the reference level (Cretaceous bottom) respectively with the excess area intercept equal to zero. However, the calculation results of depth to detachment in Yu et al (2006) are based on excess area intercept unequal to zero.