In order to investigate the relationship between the unstressed state amount and the prefabricated configuration of the plane beam element, a mathematical relationship between parameters of the prefabricated configuration and the unstressed state amount of the plane beam element was established by analyzing the geometrical configuration of the plane beam element considering the displacement of the joint. Taking a cantilever beam as an example, two types of alignment were used as inputs to verify the proposed mathematical relationship. The results show that the unstressed configuration is uniquely associated with the unstressed state amount of the element. The unstressed state amounts of different elements correspond to different prefabricated configurations. Although the prefabricated configurations of the two alignments are different, they have minimal influence on the joint elevation control when assembling the main beam segments. In engineering practice, the prefabricated configuration of the beam segment can be different as long as the joint elevation satisfies the elevation requirement of the target alignment.