Abstract: Swimming ability plays a crucial role in fish migration and habitat restoration. This paper focuses on the balance between swimming thrust and fish body resistance during fish cruising. The study considers the ‘anti-Kaman vortex street’ force generated by the caudal fin oscillation as the main source of swimming thrust, and the relationship between swimming thrust and caudal fin oscillation parameters is deduced by applying the theory of vortex motion. Additionally, the fish body resistance is divided into three intervals based on the boundary layer theory, with the consideration of either friction resistance or shape resistance depending on the flow state. The study establishes the dynamic relationship between swimming speed and caudal fin oscillation parameters, and derives the scaling law connecting the dimensionless swimming Reynolds number Re and the caudal fin oscillation number S_w, which is calibrated and verified by two-dimensional numerical simulation of fish swimming and statistics of the literature data. The results reveal that: The relationship of fish swimming Reynolds number Re and the caudal fin oscillation number S_w conforms to the scaling law of Re=k·S_\rmw^n , and the scaling law exponent n is 4/3, 10/9 and 1.00 for the swimming Reynolds numbers of 1<Re<3000, 3000<Re<10 000 and Re>10 000, respectively. The coefficient k of the scaling law is divided into three intervals with the increase of the swimming Reynolds number, which shows a negative correlation with the relative width of fish body b. The relationship of the scaling law can be further expressed as Re=k(b)·S_\rmw^n . The existence of the scaling law is proved by the statistical analysis of literature data, and the scaling law applies to fish swimming with typical carangiform modes. The study can provide support for the calculation of fish swimming speed, and then for the fish migration channel and habitat construction.