A phase-field model has been developed to study the influence of epitaxial strains and domain structures on the electrocaloric effects (ECEs) in PbTiO3 (PTO) thin films. The simulated domain-switching dynamics obtained from the proposed model was tallied with the existing experimental results. In the case of single-domain PTO thin film, the ECE gradually increased with the increasing epitaxial strain, whereas, in the case of poly-domain PTO thin film, the influence of epitaxial strain on ECE can be divided into three regions of stable domain structures, i.e., c-domains, a/c-domains, and a-domains regions. It is worth noting that the maximum ECE occurred under a tensile epitaxial strain in the a/c-domains region, which was the consequence of the competition between the enhancement of ECE and the reduction of c-domains volume fraction, both caused by the increasing tensile epitaxial strain. Although the change of temperature arising from the ECE may not be large enough to be viable for practical applications, it does illustrate the important influence of domain structures on the ECEs of thin films, which has not gained much attention of most researchers whose focus is still on single-domain thin films.