The roles of STM tungsten tip and external static electric field in single corner silicon adatom extracting from Si(111)-7×7 surface are studied using discrete variational-local density functional (DV-LDF) method with cluster models. The binding energy and the different charge density distri-bution of the system are calculated under various conditions. The results show that, the activation barrier decreasing is a chemical effect due to the procimity of the eletrodes. The effect of external electric field depends on its polarity. A monotonous decrease in the activation barrier is seen for positive bias case, while a small increasing is seen for negative bias case. The single corner silicon adatom cannot transfer from the sample surface if only considering the role of the static interaction among the tip, external electric field and sample surface. The mechanisms of atomic manipulation on semiconductor surfaces are discussed.