In the domain of integrated quantum photonics, the burgeoning superiority of lithium niobate’s second-order nonlinearity in electro-optic modulation makes thin-film lithium niobate a leading quantum photonic platform after silicon. To date, single-photon sources using thin-film lithium niobate has mainly adopted periodic polarization quasi-phase matching technology, which requires the preparation of complex electrodes for domain inversion in the waveguide to realize quasi-phase matching. This method inevitably introduces complexity, such as complex processing methods, enlarged polarization regions, and compromised integration density. With the development of quantum information technology, the ever-increasing degree of integration constantly creates new demands. Consequently, the development of a streamlined, high-efficiency quantum light source on a lithium niobate platform is a pressing issue. In this study, we propose a novel thin-film lithium niobate parametric down-conversion single-photon source based on mode dispersion phase matching theory. The strategy is different from conventional strategies that utilize periodic polarization to generate single-photon sources in thin-film lithium niobate devices. In contrast to traditional quasi-phase matching techniques that utilize the phase matching between pump fundamental mode light and parametric fundamental mode light, our method employs the phase matching between the pump light’s higher-order mode and the parametric light’s fundamental mode. The pump light’s higher-order mode is obtained by designing an asymmetric directional coupler. The device’s single-photon yield can attain $3.8\times10^{7}$ /(s·mW), satisfying the requirements for optical quantum information processing. This innovative solution is expected to replace the traditional quasi-phase-matching single-photon sources, thus further promoting the study of optical quantum information based on thin-film lithium niobate chips.