Through long- time filtering experiment in water treatment plant, indepth filtration mechanism and characteristics of the long fiber efficient filter , which has been patented with national invention, was studied in this paper. The bed structure characteristics of perfect filtering layer was showed in characteristics of the pore structure, filtrate water concentration, deposit in each layer and loss of filtering pressure along the depth of filtering bed. It was found that deposit lapse with depth of filter bed until exhaustion. It presents research direction for development of filter technique.