Causes of leftward deviation of the electrical axis of sinus rhythm can be seen in people with left ventricular hypertrophy, left anterior branch block, and long and lean body size. The normal cardiac axis ranges from -30° to +90°; an electrical axis in the range of -30 to -90° is a left-sided cardiac axis, and one in the range of +90° to +180° is a right-sided cardiac axis. The electrical axis deviation is influenced by the following factors: anatomical position of the heart within the thoracic cavity, mass ratio of both ventricles, function of the intracardiac conduction system, state of intraventricular conduction of excitation, and age and body size. Mild left-axis deviation without adverse effects can be left untreated. Regular electrocardiogram check, proper exercise and good diet are needed. For severe left-axis deviation, it is recommended to monitor blood pressure, improve cardiac ultrasound to clarify the enlargement of the left ventricle and find the cause of enlargement. If hypertension is the cause, actively lower the pressure, and if necessary, valve replacement is required for valve disease.