The progress on the study cf function mechanism of backfill for mined-out areas of underground metal mines in China and abroad are briefly described. The backfilling method is adopted and the backfill function is studied in some mines such as: Xikuangshan, Zhaoyuan, Jingchuan and Dashuiqing, Yieding remarkable econmic and social benefits. The paper also previews the development trend of this technology.