Abstract: In order to respond to the actual needs of miniaturization of traditional large instruments, based on the traditional size effect theory, uniaxial compression test is taken as the main research method. Limestone specimens with the same ratio of height to diameter and different diameters are taken as research objects to study the law of strength size effect of limestone and explore the mechanism of strength size effect. The results show that a mechanical model F=ae^(-(D-b)/c)2 is proposed, which can better characterize the size effects of the strength and modulus of limestone. The uniaxial compressive strength, elastic modulus, and deformation modulus of limestone first increase and then decrease with the increase of specimen size, which is different from the traditional size law. The peak strain decreases with the increase of specimen size, which can be expressed as a power function, and Poisson's ratio has no obvious size effect. The smaller the diameter of the specimen, the greater the dispersion of its mechanical parameters. The failure mode of the specimen is dominated by X-shaped conjugate shear failure, and the secondary failure mode changes from compressive shear failure to inverted V failure and then to single slope shear failure. It is considered that the non-uniformity of rock material is the decisive factor of the strength size effect, and the end friction effect is only the test method.