Abstract: In order to study law of surface cracks under situation of loess hilly- gully region and middle thick bedrock in extra thick seam using fully- mechanized to p coal caving method, according to surface cracks data of the test area working face, the authors studied special reason for formation of reversed steps- like cracks and formation mechanism of surface cracks, and analysed periodic characteristics of roof caving, and deduced calculation formula of influence scope for tensile cracks and shear cracks. Results showed that rate change of working face, thickness change of top coal caving and influence of land form and geological stress were the specific r easons for formation of reversed steps- like cracks. Cracks spacing along strike direction and periodic weighting interval of immediate roof were close. The calculation f ormula of influence scope for tensile cracks and shear cracks had a guiding significancefor predicting surface crack damage scope of mining under buildings.