Disclosed is an aquatic animal space size selection device. The aquatic animal space size selection device comprises a center cylinder and selection containers; the center cylinder is communicated with the six selection containers which are distributed uniformly; every selection container is formed by a cylinder R1, a cylinder R2 and a cylinder R3 which are linearly communicated with each other, wherein the radiuses of the three cylinders are different from each other; the arrangement orders of the different radiuses of cylinder R1, cylinder R2 and cylinder R3 which are arranged on every selection container are different from each other; every selection container is communicated with the center cylinder through a channel; the radiuses of the initial cylinders of every two corresponding selection containers which are located on the same straight line are as same as each other; the probabilities of appearance in the three different sizes of spaces are calculated according to the number and an experimental fish is fond of one of the spaces if the probability of the appearance in the certain space is large. According to the aquatic animal space size selection device and method, an aquatic animal breeding system can be optimized, the favorite size of space is established for bred aquatic animals, and accordingly the significant economic benefits are achieved and meanwhile the technical support is provided for the species protection.