In the sinking process of caissons of cofferdam of the Xiangjiaba Hydropower Station, the side friction and the end bearing pressure of caissons were monitored. According to the observed data, the resistance character pressure systematically was studied during the sinking, and the distribution of wall side friction and end bearing force during penetrating into different soil strata were obtained. And meanwhile, the formula of sinking coefficient KS was proposed, and from observed data, when the KS was less than 1.15 during threetime sinking, the caisson reached designed location to be difficult.