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Trioctylphosphine (TOP) is widely used in the synthetic process of colloidal quantum dots (QDs). However, its role has not been fully elucidated yet. Herein, the role of TOP was revealed by kinetic studies with spectroscopic methods. It was found that TOP suppressed the growth rate but increased the solubility; hence, it modulated QDs by controlling both. The extent of impact from TOP varied in the length of acids that was used in QD synthesis. In general, the impact from TOP to QDs synthesized by short acids was mainly the suppressing effect because shorter aliphatic acids had a higher diffusion rate, and they might suffer from TOP a lot. On the contrary, longer aliphatic acids diffused intrinsically slowly, so the reaction rate was rarely influenced by TOP, but the solubility was supposed to be considered at this time. This difference also led to the optimal amount of TOP used in the synthesis. These results pointed out that both the growth rate and solubility were time-dependent and should be considered to obtain high-quality QDs. In addition, the initial nucleation procedure was also seemingly suppressed by TOP. This work not only shed light on the underlying role of TOP and convinced the controllable synthesis of QDs but also provided a method to investigate the behavior of QD growth by considering several conditions synergistically and dynamically.