AbstractPolythioureas (PTUs) show great potentials for applications in the new generation of film capacitors due to their excellent dielectric properties. Herein, the cis‐trans copolymer chain of PTU is successfully tailored by employing cis and trans cyclohexyl spacers. The relationship between the copolymer chain conformation, microstructure, and dielectric properties is carefully explored by a series of analysis. Compared with cis conformation, the trans with less steric hindrance can promote the formation of H‐bonds. The enhanced H‐bonding interactions not only reduce the molecular inter‐chain spacing, but also drive the self‐assembly of molecular chains to form cylindrical and droplet nano‐morphologies. The phase separation between cis and trans PTUs is confirmed by combining the experimental results of TEM and DSC, and the CT64‐PTU with the most two‐phase interface thus obtains the highest permittivity of 5.5 (@10 Hz). The reduced molecular inter‐chain spacing is accompanied by a decreased hopping distance of charges, which improves breakdown strength by 17% from 498 MV/m to 580 MV/m. Therefore, the cis‐trans copolymer chain conformation in PTU provides a simultaneous high permittivity and breakdown strength. This research offers a strategy to further design high‐performance dielectrics via regulation of copolymer chain conformation.