An image analysis based method is developed in this work to quantitatively evaluate the mixing performance of a special soft-elastic reactor (SER), which promotes mixing through elastic wall movement. The source data collected are RGB values in each pixel of consecutive colour images taken by a CCD camera. The method can determine global indicators such as the average mixing level (i.e., the degree of homogeneity, uniquely characterized by the ''distance" away from the ideally mixed state) and the mixing time. More importantly, it allows tracking of localized mixing performance throughout the entire mixing process by quantifying spatial and frequency-domain distributions of the mixing level. This simple, generic, and cost-effective method should be a powerful tool for mixing performance evaluation of most reactors with a variety of mixing mechanisms. It can be readily applied in the future to improved design and optimized operation of the SER. (C) 2018 Elsevier Ltd. All rights reserved.