Central and local governments in China have issued various plans and policies to promote the development of the humanoid robot industry. Humanoid robot technology has rapidly developed recently, but some technical difficulties still exist. The quasi-direct drive actuator has the advantages of fast response speed, low cost, and high safety. It was tried to be applied to humanoid robots, but the theoretical research of its transmission transparency was not enough, which affected its large-scale industrialization. The research status of transmission transparency of quasi-direct drive actuators for humanoid robots at home and abroad was analyzed, and the definition of transmission transparency was proposed. The model of transmission transparency of a quasi-direct drive actuator was established, and its influencing factors were explored. The evaluation index of transmission transparency of the quasi-direct drive actuator was proposed, and the experiment scheme and method were designed. This investigation was significant for promoting the technological progress and industrial landing of quasi-direct drive actuators.