Graphic statics consists of interactive force diagram and form diagram. Form diagram generated by force diagram plays a very positive role in structural form design in protocol stage. Firstly, this paper explores that the prototype of force diagram should have an appropriate abstract level, which is moderately abstract concept representation according to the theory of cognitive psychology. Secondly, it constructs its 2D lineage from 5 aspects, e.g. the direction of structural internal force, the number of supports, the component form, the load distribution and the load decomposition. At last, this paper determines the number of parameters and figures out the relations between parameters and writes a program to generate many structural forms. This paper aims at the improvement of interaction level, which makes it easier to operate force diagram when architects conceive projects.