It is very important to determine and master the exercise intensity in exercise. Appropriate exercise intensity can effectively improve the body function. Too much intensity can make the body function decline, and even damage the health. Aiming at the problems of few assessment indicators and low accuracy of traditional exercise intensity assessment methods, this paper proposed an exercise intensity assessment method based on knowledge graph. Firstly, the indications of exercise intensity assessment were extracted by BERT-CRF, and the knowledge graph of exercise intensity assessment was established in the Neo4j database. Secondly, on the basis of RecGNNs knowledge reasoning, personalized and accurate assessment of exercise in-tensity for different age groups was realized. The exercise intensity assessment system based on this method can display the user's exercise data index and personalized exercise intensity assessment results. Finally, the exercise intensity assessment system was constructed. The experimental results show that the exercise intensity assessment system based on knowledge graph was stable and reliable, and the accuracy of personalized assessment can reach 85.7%.