Molecular semiconductors (MSCs), characterized by a longer spin lifetime than most of other materials due to their weak spin relaxation mechanisms, especially at room temperature, together with their abundant chemical tailorability and flexibility, are regarded as promising candidates for spintronic applications. Molecular spintronics, as an emerging subject that utilizes the unique properties of MSCs to study spin-dependent phenomena and properties, has attracted wide attention. In molecular spintronic devices, MSCs play the role as medium for information transport, process, and storage, in which the efficient spin inject–transport process is the prerequisite. Herein, we focus mainly on summarizing and discussing the recent advances in theoretical principles towards spin transport of MSCs in terms of the injection of spin-polarized carriers through the ferromagnetic metal/MSC interface and the subsequent transport within the MSC layer. Based on the theoretical progress, we cautiously present targeted design strategies of MSCs that contribute to the optimization of spin-transport efficiency and give favorable approaches to exploring accessional possibilities of spintronic materials. Finally, challenges and prospects regarding current spin transport are also presented, aiming to promote the development and application of the rosy and energetic field of molecular spintronics.