In order to extend the usage range of a cascade having excellent aerodynamic performance, it is beneficial to investigate the similarity between different flow fields. Von Karman gave transonic similarity law of two-dimensional isolated airfoils many years ago. However, the law of cascades is still different from that of airfoils. This paper points out that, to guarantee similarity between two flow fields around cascades, it is necessary that five corresponding transonic similarity parameters must be kept equal. Also some examples have been presented in this paper for demonstration. They indicate that the similarity law will help us to obtain rapidly many similar transonic flow fields around cascades at different operating conditions from a known flow field around a given cascade.