Abstract: In this paper,a new method of the focal plane imaging for measuring liquid diffusion coefficient(D)quickly and accurately is described systematically.With the unique spatially resolving ability of liquid-core cylindrical lenses (LCL)for measuring refractive index (RI),D is measured by Fick’s second law.In order to improve the measurement accuracy of D,the LCL must own high refractive index sensitivity,small measurement error of focal length,and the small measurement accuracy of n value;at the same time,the spherical aberration(SA) should be reduced to ensure the imaging quality of the diffusion image.According to different needs,we designed and fabricated glass capillary,symmetric liquid-core cylindrical lens (SLCL),asymmetric liquid-core cylindrical lens (ALCL) and double liquid cylindrical lens (DLCL) respectively,for measuring liquid diffusion coefficient(D)quickly,accurately and stably.The measuring instrument based on the method of focal plane imaging can be developed into a D measuring instrument with complete independent intellectual property rights.