The size effect of red bed soft rock's uniaxial compressive strength was researched in this paper.The testing samples are taken from Humaling Tunnel of the Xiaguan-Guangyuan Section of Lanzhou-Chongqing Railway,Dingxi city,Gansu Province,China.The samples are of cylindrical shape with different ratios of height to diameter(diameter 50mm,height 20-100 mm).Results show that the red bed soft rock has obvious size effect.The dimensionless size effect coefficient is defined as the ratio of uniaxial compressive strength of nonstandard sample to that of standard sample.Fitting of test data shows that size effect coefficient is linearly related with height to diameter ratio and average elastic modulus.A mathematical model is established to illustrate this linear function.When the height to diameter ratio is smaller than 2.0,the size effect coefficient decreases and then increases with the increase of the ratio.The height to diameter ratio corresponding to minimum coefficient is obtained.Moreover,stress analysis is conducted to research the destruction patterns of red bed soft rock,and the causes of size effect coefficient variation are given.