Abstract: Pipeline Inspection Gauge (pig) is an important equipment for oil and gas pipelines during different stages of their operations to perform functions such as dewatering, cleaning, and inspection. Owing to the hyperelasticity, time and temperature-dependent material behaviour of the sealing disc attached on the pig, the contact between the pig and the pipeline expresses complex behaviour, leading to an uncertainty in the prediction of the pig's frictional force. Knowing the deformation of the sealing discs well is essential and can be highly meaningful for predicting the pig motion, as well as reducing the pigging risks. In this study, the geometrical deformation of the sealing discs with different sizes are investigated through experiments and numerical simulations. The effects of the four nondimensionalized parameters (interference, thickness per pipeline inner diameter, and clamping ratio) of the sealing discs on the deformation behaviour were observed and discussed, and an improved mathematical model for predicting the geometrical deformation of the sealing discs was proposed and verified. With the auxiliary angle α added in the improved mathematical model, the relative error declines to 1.87% and 3.18% respectively for predicting deformation of the sealing discs in size of 2-inch and 40-inch pig. The results of this study can help better understand the frictional force of a pig with sealing discs, as well as its motion.