Effective cover rate for soil and water conservation forests is an important index in the construction of forestry. This terminology appeared early, but its value are fluctuating from 30%to 75% for there is not a definite concept and the same method for determining it. In view of this, based on analysis of the system of soil and water conservation forest in detail, the concept of the effective cover rate for soil and water conservation forest is advanced as the cover rate when the amount of soil loss is equal to the permissible amount of soil loss in a given range: ae(-bF)=SLA. And its determining method is concerned with two important aspects and three steps, the effective cover rate is a function of time and space. Finally the soil and water loss region in Loess Plateau was exampled to introduce the determining method of effective cover rate.