Transition metal dichalcogenides (TMDs) and their heterostructures exhibit excellent optoelectronic properties, thus rendering them promising candidates for next-generation electronic and optoelectronic devices. Investigating the ultrafast carrier dynamics in these materials not only allows for the exploration of ultrafast optoelectronic conversion processes within new materials but also provides a theoretical foundation for the development of high-speed optoelectronic devices. This review focuses on the latest advancements in carrier-dynamics behavior in TMDs and their heterojunctions. An optical pump-terahertz probe (OPTP) system and optimization methods were introduced to achieve OPTP signals with large signal-to-noise ratios in nanomaterials. Based on analysis using the OPTP, we summarized the different physical processes involved in the relaxation of TMDs, including exciton dynamics, phonon-assisted recombination, Auger recombination, and defect-induced relaxation processes. Aditionally, we discussed charge transfer via the OPTP in TMD heterostructures tuned by the pump wavelengths and stacking orders. This review summarized the recent progress of ultrafast dynamics in TMDs based on an OPTP and indentified some potential directions in condensed matter physics.