The concept and calculation method of the dimension of distribution system security region(DSSR)are proposed. Firstly

the DSSR dimension is defined when the number of both constraints and variables are considered synthetically. Secondly

a DSSR dimension calculation method based on the main transformers connection vector is proposed. The key point in this method is to define a vector to describe the connection relationship among the main transformers and the DSSR dimension can be obtained without listing the security region expressions. Finally

an example is given to illustrate the proposed method. The relation between DSSR dimension and the distribution network is acquired by the preliminary analysis. The condition where the number of boundaries reaches the extreme value is also obtained. For different scale of distribution networks

the DSSR dimension is mainly affected by the number of transformers

feeders and sectionalizing switches. For the same scale of distribution networks

DSSR dimension may be different

and it is mainly affected by the connection modes of feeders. The concept of DSSR dimension proposed can rigorously describe the scale of DSSR and its corresponding N-1 security problems

which provides a necessary foundation for DSSR theory.