How to determine the length of power-injected pile is a important problem in practical project. Considering the complex special distribution of soft earth stratums in fact, the length of pile by prospecting will be unsatisfied to the requirement of improvement. The author participated in a project of improvement of soft ground by power-injected pile, put forward controlling the length of pile by the intensity of working current, put it in practice and succeeded considerable technical and economic benefits. The experimental data and conclusion from engineering project is valuable to similar projects.