Nine-year-old girl’s heartbeat is too fast may be emotional tension, hyperthyroidism, arrhythmia and other reasons. 1. Emotional tension: emotional tension will make the body sympathetic nerve excitation, the role of the sympathetic nerve receptors in the heart lead to increased excitability of the heart muscle, so the heart beats too fast. In addition, overexertion, strenuous exercise leads to an increase in myocardial oxygen consumption can also lead to a fast heartbeat. 2. Hyperthyroidism: hyperthyroidism leads to an increase in the synthesis and secretion of thyroid hormone, which acts on the cardiac muscle cells and leads to hypermetabolism of the myocardium, an increase in myocardial contraction force, and a faster heart rate. 3. Arrhythmia: various organic and non-organic lesions lead to heart excitation, conduction block dysfunction, leading to changes in the normal rhythm of the heart, heart rate faster. A nine-year-old girl’s fast heartbeat may be related to a variety of factors, and it is recommended to seek medical attention for a clear diagnosis and active treatment.