Augmented reality (AR) offers potential advantages for intensifying environmental context awareness and augmenting students' experiences in real-world environments by dynamically overlapping digital materials with a real-world environment. However, some challenges to AR learning environments have been described, such as participants' cognitive overload and the ways to provide assistance in constructing the presented learning materials. In this study, a mindtool-based AR learning system was developed, based on the repertory grid method and the contiguity principle of multimedia learning, for assisting students in constructing their knowledge in a natural science course. Furthermore, an experiment was carried out on an elementary school natural science course to compare the influences of this method with those of the conventional AR learning system on students' learning effectiveness. The experimental results show that the designated approach effectively promoted the students' learning achievements, and no significant difference existed between the mindtool-based AR learning system and the conventional AR learning system in terms of students' cognition load and satisfaction degree; moreover, both the experimental group and the control group perceived low cognition load during the learning activity and rated their own AR learning systems as being highly satisfactory.