Abstract: The relational schema that is permited to have null value is the basis of setting up in- complete information database.In order to further discuss the problems such as decomposing of this kind of relational schema,we have to reserch deeply about minimal cover problem for set of func- tional dependancy under null value circumstances at least.We discuss the problem in this paper,also we give out a solution of solving minimal cover of functional dependency set that satisfied data ho- lding conditions under null value circumstance,so prepared the condition for the deeply reserching of the relational schema that is permited to have null value.