The shakedown study for determining plastic shakedown limit is desirable for preventing geotechnical infrastructure failure under cyclic loads. The plastic shakedown limit is conceptually clear but difficult to determine experimentally since the permanent strain accumulation response is a gradual process, and the criterion for determining the plastic shakedown limit is sensitive and variable limited by lack of further understanding of mechanisms during shake down process. Motivated by the need to clarify the mechanism of shakedown limit, a new concept of elastic upper limit of quasi-elasticity extension is hypothesized in this paper. Three-stage slow cyclic triaxial tests are carried out on three types of sand to clarify this new concept. The results show that the quasi-elastic curve extends as the cyclic stress level increases until it reaches the elastic upper limit and then the deviation occurs. The objective of this study is to explore the role of elastic upper limit (the elastic extension domain limit) in the shakedown theory. It is found that the derived elastic upper limit is close to the plastic shakedown limit, especially at high levels of confining pressure. A new criterion is proposed to determine the plastic shakedown limit based on the linear dependence of the degree of elasticity on the cyclic stress level. The introduction of the concept of elastic upper limit provides new insights into shakedown study for granular soils.