A new thermal-mechanical coupled numerical model was proposed based on the lattice discrete element method (LDEM), aiming at the heat conduction and thermal cracking of quasi-brittle materials. In LDEM, in order to simulate the numerical test of the temperature conduction and crack propagation, the equivalent conversion of the heat transfer process between the lattice discrete system and the continuum was carried out, combined with the linear expansion formula of the lattice element. Taking the heat conduction and thermoelastic stress of the plate, and the cracking caused by the temperature gradient and thermal mismatch as examples, the model was verified. In addition, the model was applied to the numerical simulation of the meso-level concrete temperature-stress test. Results show that the LDEM thermal-mechanical coupling model can simulate the heat conduction process of quasi-brittle materials, as well as the crack initiation and propagation under the influence of temperature effectively, which provides a powerful tool for studying the thermal cracking process and mechanism of quasi-brittle materials.