Studies of tool affordance effect for the last two decades have changed the understanding of perception. This article reviews the important findings and discusses different explanations of affordance perception by traditional cognitive psychology and ecological psychology. We analyze why traditional approach is inadequate to explain affordance perception, and how resonance theory can offer a better interpretation from ecological perspective. As argued by Gibson, affordance is an emergent action possibility in the animal-environment system. It is a relationship between animal and environment about what actions an environment affords and an animal can adopt. Empirical evidence shows that human can immediately perceive the affordances of a tool without subjective intention. Physiological observations demonstrate that neurons in premotor cortex and dorsal stream active remarkably as soon as the subjects see a tool in reachable distance, even the subjects do not need to take any action. Furthermore, human are able to perceive affordances of more than two tools simultaneously, and also to perceive affordance for others. Traditionally, cognitive psychology proposes that the perceptual system first collects low-order sensory information to build an internal descriptive representation of objects in the external world. And then, this information is used along with representations of current needs and memories of past experience to deduce high-order information, such as affordance, and make judgments and decide upon a course of action. However, such claims have encountered difficulties in interpreting neural activities in terms of distinct perceptual, cognitive or motor systems, as the motor system immediately actives when seeing a manipulable and reachable tool.? To response the challenge, some traditional psychologists propose a motor simulation theory and suggest that when seeing a tool, brain also represents the actions and operations related to it, and that we simulate these actions, estimate actions possible and suitable, and predict the results of these actions. This is claimed as the way in which human perceive affordances in environment. The motor simulation theory has made a good progress in robot design but it is not as nearly good in explaining affordance perception, since the theory is inadequate to explains the findings: (1) Affordance is not real action but action possibilities about what animals can do and cannot do, and the motor simulation theory largely disregards the distinction. (2) Experiments show that individuals can perceive affordance without simulation. (3) Motor simulation theory confines affordance perception within tool perception, does not take into account that affordance perception takes place far beyond the domain and may extend to much broad relationship between human perception and environment. Building on animal-environment system, ecological psychology explains affordance perception differently. According to ecological psychology: (1) Affordance is an emergent reality in animal-environment system, not knowledge existed in our head. (2) Affordances are what we perceive, and what we attend first in the environment, and, as many experiments that compare visually extinct patients with normal vision participants have demonstrated, the priority of affordance in perception. (3) During evolution and learning, individuals adapt themselves to different affordance information, and such adaptations bring changes in body state, such as activation of motor neurons and muscle cells, transformation of actions. (4) When individuals meet environmental affordance information, they pick up the information without retrieval of memory, and the direct perception of affordances appears to be an outcome of the resonance between our body’s motor system and affordance information in the environment.