In order to study the disintegration characteristics of red-bed soft rock in Xiangtan, Hunan province, the indoor disintegration tests of dry and wet cycles were carried out on four different mass samples under the simulated rain conditions. In addition, the calculation method of fractal dimension (DV) based on the correlation between the volume and the particle size of the rock was deduced and then used to study the disintegration characteristics of the samples after the disintegration tests. The results show that the curve shapes of the relationship between the cumulative percentage and the particle sizes of the samples after disintegration are very similar, which are uniformly convex. With the increase of the number of dry and wet cycles (N), the large particles decrease, the small particles increase gradually, and the grading curves of disintegrated red-bed soft rock tends to coincide, reflecting that the disintegration rate of the samples slows down gradually. Furthermore, the convex part of the curve becomes more and more prominent with the increase of sample mass in each group. With the increase of N, the DV of different sample mass increases rapidly at first, then slows down after the fourth cycle, and finally stabilizes. Besides, with the increase of the sample mass, the DV of the samples increase gradually, indicating that the larger the sample mass is, the faster the disintegration rate will be. This phenomenon can be attributed to the influence of sample size effect on its disintegration characteristics.