Variable-displacement vane-type oil pumps represent one of the most innovative pump types for industrial applications, especially for engine lubrication systems. The aim of this paper is to develop a complete and accurate mathematical model for a typical variable-displacement vane-type oil pump to investigate its working performance. First, the detailed theoretical model was built on the basis of pump geometric design and dynamic analyses. Next, numerical simulations with the constructed model and experiments on the actual pump system were carried out to analyse the main power loss factors in order to develop the complete model for high modelling accuracy. The estimated pump performance using the complete pump model was finally verified by numerical simulations in comparison with practical tests.