The stretch blow moulding (SBM) process is the main method for the mass production of PET containers. Understanding the constitutive behaviour of amorphous PET (aPET) during the blowing process is critical for designing the optimum product and process. However due to its nonlinear viscoelastic behaviour, the behaviour of aPET is highly sensitive to its thermomechanical history making the task of modelling its constitutive behaviour very complex. This means that the constitutive model will be useful only if it is known to be valid under the actual conditions of the SBM process of interest. In order to characterize the constitutive behaviour of aPET during the blowing process, digital image correlation was employed to capture the actual deformation of material, thereby obtaining the corresponding stress-strain relationship by replicating this strain history via a biaxial stretching tester. Furthermore, an upgraded, in-situ characterization methodology based on both digital image correlation and implemented instruments was developed, which provides more efficiency and flexibility of determining the constitutive behaviour of materials during the blowing process.