A method to measure liquid diffusion coefficients based on double liquid-core cylindrical lens (DLCL): equivalent observation altitude measurement method is introduced. The front liquid core of DLCL is used as both diffusive pool and main imaging element, and the rear liquid core is used as aplanatic auxiliary element. A fixed altitude of DLCL is selected as observation position. Based on the Fick’s second law, the liquid diffusion coefficient is calculated according to the relationship between the diffusion image width and time. The diffusion coefficient of 0.33 mol/L KCl aqueous solution is D=1.8530×10 -5 cm 2/s, which is measured at the room temperature (25 ℃) and by equivalent observation altitude measurement method. The relative error between the measured results and the literature values is 0.65%. The proposed method makes use of the advantage of DLCL that can reduce the spherical aberration in certain range of refractive index, which makes the liquid diffusion coefficients have the characteristics of high accuracy (relative error is less than 1%), high speed (measurement time is about 60 min), and the visualization of diffusion process.