Given that traditional estimation strategies often focus on a single condition, the present study firstly investigated the strategies and cognitive mechanisms employed by children in an alternating presentation of the number line estimation task. In Experiment 1, second-grade children were asked to estimate numbers within different ranges but with the same length (0~50, 0~100, 10 cm), while in Experiment 2, they were asked to estimate numbers with different lengths but the same range (10 cm, 18 cm, 0~100). The results revealed that children utilized multiple strategies, employing both internal reference strategies and external reference strategies based on the proportional relationships between conditions. Additionally, the interaction between presentation format and experimental context was found to have a significant effect on the proportional absolute error (PAE). Due to limited cognitive resources, children exhibited significantly better strategy performance in the alternating presentation condition compared to the concentrated presentation condition when dealing with different ranges. However, their strategy performance was significantly worse in the alternating presentation condition when dealing with different lengths. These findings suggest that second-grade children’s strategies exhibit a pattern of “coexistence and competition”, providing new evidence for the applicability of the overlapping waves theory in complex estimation contexts.