The ultrasonic spot welding of fibre-reinforced thermoplastic laminates has received great interest from researchers, mainly in the fields of aerospace and automotive industries. It offers an efficient solution to join large thermoplastic composite parts through the spot welding approach with a high level of automation. In this paper, the temporal and spatial development of the temperature in an ultrasonic weld spot between two fibre-reinforced thermoplastic laminates was modelled. During the ultrasonic welding of thermoplastic composite laminates without energy directors a sudden temperature jump in the weld spot is usually observed. The temperature increase occurs rapidly up to the decomposition of the thermoplastic matrix and causes the degradation of the weld spot. To understand the temperature distribution within the weld spot and to calculate its temporal development, the thermal problem was analysed using a two-dimensional explicit finite difference method. To evaluate the models, the calculated time traces of the temperature in the weld spot were compared with the experimentally obtained values under comparable conditions.