Over the last decade, considerable attention has been paid to the formation mechanism of the ordered β conformation for PFO both in the solution and film to prepare high-efficiency optoelectronic devices. However, the process of solvent evaporation and aggregates transferred from the solution to the film also play key roles in forming ordered structures, which have been neglected. In this study, the influence of molecular weight on the above process was systematically studied using techniques such as SLS/DLS, UV-Vis, PL, and TEM. Five samples with different Mw ranging from 25 100 Da to 117 000 Da were obtained by the precipitation fractionation method. In dilute THF solution, the molecular chains were in α conformation without aggregates. In films, as the molecular weight increased, the content of β conformation and ordered structure increased first and then decreased. By studying the solvent evaporation process, for the first time, we propose a possible mechanism for the transformation process of chain structure and β conformation from the solution to the film, which involves three stages. This study reveals the transformation process of the chain structure and β conformation of PFO from the solution to the film and its relationship with the molecular weight, which provides theoretical and practical versions for in-depth understanding and control of the formation of the ordered structure in high-efficiency films.