The study on swimming ability of shrimp is of great significance to the regulation of flow velocity and duration in the industrial recirculating aquaculture system. In this study, the swimming time of 3 sizes [S1 (5.87 ± 0.53), S2 (10.12 ± 0.62) and S3 (14.28 ± 0.12) g] of Litopenaeus vannamei was measured in circulating flume at 6 kinds of flow velocity (29.4, 32.9, 37.5, 41.2, 44.3 and 50.3 cm/s), and taken as the index to measure the swimming ability of Litopenaeus vannamei. The results showed that with the increase of flow velocity, the swimming time of all sizes of shrimp showed a decreasing trend, and the decrement increased with the increase of the size. Under the same flow velocity, the larger the shrimp size, the shorter the swimming time, with obvious individual variation; the coefficient of variation was used to characterize the individual variation. The coefficient of variation showed an overall upward trend with the increase of flow velocity for all the 3 sizes of shrimp. Among them, the individual variation of shrimp of S3 was obviously greater than that of shrimp of other sizes; the shrimp size basically has no effect on the selection of the mathematical fitting model of average swimming time - flow velocity, and the logarithmic model fitting coefficient of the three sizes of shrimp is the highest. The results can provide a reference for the regulation of flow velocity for shrimps of variable sizes in industrial recirculating aquaculture system.