Understanding and controlling the morphology of conjugated polymers is crucial for optimizing device performance in various applications such as stretchable electronics, wearable sensors, and flexible displays. Manipulating the morphology of such polymers into specific nanostructures like nanofibers or nanowires can not only maintain their electrical properties but also enhance mechanical robustness, enabling them to withstand mechanical deformations such as stretching, bending, or twisting without compromising their electrical functionality. While the majority of research on controlling conjugated polymers has focused on p-type materials, establishing an approach to precisely control diverse conjugated polymers, especially for n-type conjugated polymers, into such morphology remains a significant and elusive challenge in the field. In this work, we design a new n-type semiconducting polymer and present a pre-aggregation controlling approach that enables the morphology of the novel n-type conjugated polymer to transition from fibril into nanofibers. Utilizing the Hansen solubility parameter, we systematically screen thousands of solvents to refine our selection, thereby providing a comprehensive guideline for selecting solvents conducive to nanofiber formation. We visualize a shape change in polymer pre-aggregation within the solution state upon the addition of different anti-solvents, driven by their interaction affinities with the polymer's backbone or alkyl chain. We further establish a self-assembly nanofiber formation mechanism, discussing the effects of solvent/anti-solvent interaction and the influence of boiling points on this process and draw correlations between the pre-aggregation in the solution state and the morphology of the solid state. By extending our rules to p-type conjugated polymers, we successfully tune them into nanofibers, thereby validating our mechanism and demonstrating the universality of our guideline. The demonstrated n-type semiconductor nanofibers will offer deeper insights into the development of complementary circuits and sensors. The ability to control the morphology of conjugated polymers offers enhanced avenues for advancements in flexible and stretchable electronics and paves the way for the development and design of materials with tailored properties for next-generation electronic applications.