Formal context is the basis of concept lattice theory. And it largely determines the scale and accuracy of the expression of domain knowledge. In this paper, the knowledge of completeness and reduction in formal context is mainly discussed, jointly considering the different hierarchies relationship between objects and attributes in concept lattice. The attribute of different importance is handled differently. The theory and methods about the approximation reduction of formal context is proposed. The degree II of gauge theory dealing with the redundancy is also prospected, depending on which the way to solve the missing-value context is analyzed.