Abstract: When the floor slope angle of ore body is deficient,it often adopt the method of digging floor rock in order to reclaim rudimental ore between two drifts with adopting drilling through vein non-pillar sublevel caving.On the background of ore body condition of Zhangjiawa Iron Mine,the paper has analyzed the changing law of rate of ore recovery,rate of rock drop-in,mined ore grade and productivity of extractive ore in the vary height of digging rock,has discussed the changing characters of digging rock height when ore price and mining cost changing,has educed critical height of digging rock and the bourn value of digging rock of Zhangjiawa Iron Mine in the currently condition.