A collapse accident occurred in large deep foundation pit of X power plant circulating water pump house in V country.To find the reasons,we use the calculation method of “equivalent beam” and“m” to calculate the force state of the supporting structure.The calculation principle of the two methods is described. Based on this project,we carry out the calculation.The results show that the maximum stresses of the supporting piles are more than the allowable stresses of the steels,which is the inevitable reason of the collapse accident.