Structural relaxation is one of the most important features of a glass material at an elevated temperature. This study aims to obtain an insight into the structural relaxation process of a typical optical glass. Experimentally, a high-temperature impulse excitation technique was used to explore the mechanism through monitoring the variation of the Young's modulus of the material. Theoretically, the Tool–Narayanaswamy–Moynihan (TNM) model was employed to describe the variation of modulus with time and temperature. It was found that the parameterized TNM model obtained by modulus data could be used to achieve a more subtle description of the variation of coefficient of thermal expansion during structural relaxation, which is vital for thermoforming a glass product.