Abstract: Accurate assessment of back tension crack depth is very important because it dictates the accuracy in slope stability analysis. In this paper, we reviewed three methods based on the existing literature and provided a brief introduction to the model for derivatives of these formulas. Although three formulas take different forms, they produced the same results. Based on analysis we recommended to use two of them in slope stability analysis. With mathematical analysis of the stripe piece model we determined the minimum extreme height of the strip. The obtained extreme height of the tension crack is equal to those results from three methods. The dip angle of the bottom of the stripe piece is the maximum angle of the sliding surface in the back. From the perspective of mechanical analysis, the slope body behind the stripe with a minimum height can be a stable body that sustains the tension force. The above analysis further our understanding of the condition and mechanism of the tension crack.