With the growing demand for robotic operation in unstructured environments, fixed-configuration robots are increasingly limited in adaptability, fault tolerance, and task versatility. Modular self-reconfigurable robots, composed of standardized homogeneous or heterogeneous modules, can reorganize their topology to achieve morphological and functional reconfiguration. Since existing studies are commonly classified by geometric configuration or connection topology and therefore do not fully reflect the essential differences in reconfiguration mechanisms, this paper reviewed modular robots from the perspective of reconfiguration principles. They were categorized into four groups: non-self-reconfigurable, mobile self-reconfigurable, pose-transformation-based (translation/rotation) self-reconfigurable, and joint-motion-based self-reconfigurable systems. The mechanical principles, representative prototypes, control characteristics, and application scenarios of each category were systematically summarized, and their respective advantages and limitations in compactness, mobility, control complexity, energy efficiency, and environmental adaptability were compared. This review provides a reference for configuration design, strategy selection, and application optimization of future modular self-reconfigurable robots.