The particles in the pipe surge type non-cohesive soil were divided into three groups: skeleton particles, blocking particles and movable particles, and on the basis of analyzing the force of movable particles in the skeleton pores, the total pressure drop was divided into three parts: the pressure drop caused by the skeleton pore wall, the pressure drop caused by the static particles in the skeleton pores and the pressure drop caused by the moving particles, so as to obtain the critical head gradient formula for the starting of movable particles. The verification of the pipe surge test confirms the effectiveness of the formula. The calculation results show that when fine particles are lost, the discontinuous pipe surge soil is more likely to develop into pipe surge failure than the continuous pipe surge soil. At the same time, it is also confirmed that with the loss of movable particles, pipe surge failure can be transformed into flow soil failure.