With the development of Internet of Things (IoT) technology, more and more IoT sensors are used in environmental monitoring, logistics management, manufacturing, medical care and other fields. The service time of these IoT sensors depends on the service time of the internal battery, which is not removable. Once the battery is damaged or the power is exhausted, it needs to be replaced with a new IoT sensor which greatly increases the cost. The development of energy harvesting technology provides a novel direction for solving the problem. Eliminating the need for batteries, the technology collects weak energy from the environment and converts it into electricity to power IoT sensors through related technologies. This paper summarized the key technologies of the energy harvesting system, compared the advantages and disadvantages of different circuit structures, and summarized the maximum power tracking technology (MPPT). It shows that MPPT technology has developed from one-dimensional MPPT technology to three-dimensional MPPT technology, and now more assistive techniques have been developed to improve the tracking accuracy of MPPT circuits. In order to achieve more extensive applications, multiple-input and multiple-output converters have become the focus of research. In this paper, the converter structure of multi-energy-source-input and multi-output-voltage was summarized, and the performance indexes such as static power consumption and dynamic range became the focus of attention.