In the three － dimensional design of transmission lines，the digital model of the tower is constructed by parametric modeling． The structure and geometric information of the tower is expressed by MOD files． However，the tower is a spatial structure composed of plates and bars with a certain width and thickness，the MOD file of the tower can only express the node number，coordinates，bar specifications，material，limb orientation，hanging point attributes and coordinate information． The relative position relationship between members lacks of normative description． Therefore，the format and information of MOD files need to be expanded and deepened to meet the future needs of the tower digital model． Firstly，the data structure of MOD files of the tower is introduced，and then the shortcomings of MOD files are analyzed from the point of view of adding descriptive information of relative position relation of members． Finally，a new method which can express the relative position relationship of the member is proposed．