To achieve carbon neutrality in the context of the 'Carbon Peaking and Carbon Neutrality' target, thermal recovery methods need to be improved. solvents and steam co-injection can improve the performance and greatly reduce energy consumption. As one promising solvent, Dimethyl ether (DME) has merits like low corrosiveness and certain solubility in both oil and water. However, the diffusion coefficient of DME in crude oil is still unclear. To this end, a diffusion model of DME was established based on the pressure-decay diffusion experiments at high temperature and high pressure, combined with Fick's second law, PR-EOS, solubility of DME and the mixture viscosity data, etc. By discretizing the entire model in which diffusion takes place into a number of control volumes, the parameters in each control volume were iterated one by one, and the empirical relationship of the diffusion coefficient, DAB = kTμ-β was fitted. After that, the effect of DME on the steam-solvent co-injection was analyzed. The results show that: (i) the empirical constant k of DME in oil sand at 140 ℃ and 200 ℃ are 1.15×10-12 and 1.95×10-12 respectively when β = 0.46 is used in fitting experimental data；(ii) the diffusion coefficient of DME in oil and oil sand at high temperature and high pressure range in the order of magnitude of 10-10 m2/s. Elevated temperature increases the diffusion coefficient while porous medium slows down the diffusion process. (iii) The diffusion depth of DME is several centimeters. So the drainage area would be divided into solvent diffusion-temperature dominated area and temperature dominated area；(iv) The contribution of DME to the oil production is about 11%, which is more significant for recovering high viscous reservoirs. In this paper, a set of low-cost and easy-to-implement method is established to calculate the diffusion coefficient of DME, which is confirmed to be reliable by fitting experimental data, and can accurately quantify the concentration distribution of DME in crude oil at a specific temperature, which deepens the mechanistic understanding of mass and heat transfer at drainage front, and provides theoretical guidance for the process of steam-solvent co-injection.