Leftward deviation of the electrical axis is a term commonly used in ECG findings to indicate a change in the electrical vector of the heart when the central electrical axis is changed. The electrical axis is the long axis of the heart and refers to the line between the base of the heart and the heart. Leftward deviation of the electrical axis is a counterclockwise change of the electrical axis, which can occur in physiologic or pathologic conditions. Physiological changes are often seen in obesity, the heart is in the transverse position, the apical part of the heart is shifted to the left upward, which drives the left deviation of the electrocardiographic axis; pathological state mainly occurs in the patients with left heart hypertrophy, the left atrium or the left ventricle is mainly seen, the total amount of electrical excitation activity of the left side of the cardiac myocytes after the occurrence of hypertrophy increases, which is manifested in the ECG as an increase in the amplitude of the fluctuation, resulting in the left deviation of the electrocardiographic axis, and the appearance of the anticlockwise transposition.