The invention discloses a method for determining a spare target point for a tower mirror field. The method comprises the following steps that firstly, the mirror field is partitioned; secondly, a mirror field boundary point is determined by using a boundary line of each mirror field partition; thirdly, the radius of a hazardous area is calculated; fourthly, two circle tangents crossing the mirrorfield boundary point are determined; and fifthly, the intersecting point of the two circle tangents is worked out, and the spare target point corresponding to the area is obtained. Compared with the prior art, the method has the beneficial effects that the spare target point is determined by the adoption of the method, it can be ensured that a heat absorber is rapidly put into and withdrawn from the mirror field, it also can be ensured that the heat absorber and equipment near the heat absorber operate safely when the power is lost, and an important role is played in the operation process of atower power plant.