The radial measurement method of the axial displacement of the rotor often suffers the changing shafting structure and the complicated installation and debugging of the rotor. In light of this problem a measurement method based on the single point pulse width variation is proposed. The axial displacement of the rotor is calculated by the bar code geometry， rotor operating parameters and the pulse width before and after the axial displacement of the rotor is measured by the optical color-coded sensor. First， the basic principle of measuring axial displacement by radial pulse width method is derived. The axial displacement and pulse width calculation model of the triangle print bar code considering the contour error is established. Then， the influence of the bar code structure parameters and the rotor condition conditions on the test results is analyzed. Finally， the effectiveness of the method is verified by the test platform of the precision rotor bearing system with air bearing support. The results show that the measurement error is greatly affected by the tilt angle of the bar code. When the rotor diameter is large， the method maintains the accuracy by increasing the bar code width and reducing the number of bar codes. The method provides a simple and easy new idea for the radial monitoring of the axial displacement of the rotor without changing the shafting structure.