The herd behavior acted by consumer is a typical phenomenon in consumption market. It refers to the behavior that consumers follow others’ decisions under the uncertain situation. This research explored the psychological mechanism of herd behavior by simulating situations of consumption. First experiment studied the way of information processing by manipulating the quality of message. The result shows that the herd behavior is on account of consumers using consensus heuristic and competence heuristic. The second experiment tested the condition that two heuristics were applicable. The result shows that when consumers face a high degree of consensus, they would like to use the consensus heuristic; but when consumers face a low degree of consensus, they would like to be in agreement of the competence heuristic.