CO2 sequestration in deep and unmineable coalbeds is regarded as a viable option for carbon storage. CO2 diffusion is a key assessment parameter to effectively seal CO2 in the coal seam. By conducting laboratory tests of coal CO2 desorption and unsteady gas diffusion theory model calculation, the desorption dynamic characteristics of Yaojie coal was studied under the condition of constant temperature and different balance pressure, and the CO2 diffusion coefficient was obtained through the non-steady diffusion model at different time. The results show that the higher the balance pressure, the greater the diffusion coefficient under the same condition. As the pressure drops, the CO2 diffusion coefficient gradually decreases as time increases, and the CO2 desorption mass fraction has a linear relationship with the diffusion coefficient. It is observed that under high pressures, the diffusion coefficient initially increases, then gently decreases. The non-steady CO2 diffusion in Yaojie coal relates to the CO2 swelling effect of the coal, especially in high pressure condition, the glass transition of coal with CO2 maybe the governing factor leading to the nonlinear diffusion of CO2 in coal.