Researches are carried out on the mechanical properties of red sandstones treated at 20 °C, 70 °C, 140 °C、200 °C, 300 °C, 400 °C, 600 °C and 800 °C, respectively. Based on the researches, the regular patterns of the pore structure and micro fracture of the heated red sandstones are investigated combined with mercury and scanning electron microscope analyses. It is concluded that: (1) the maximum uniaxial compressive strength of heated red sandstones at 300 °C is 1.4 times as much as that of red sandstones at room temperature. The uniaxial compressive strength decreases by 32.5% when red stones are heated to 600 °C. (2) The crack widths of heated red sandstones are mainly ranged from 0 to 0.01 μm, during which there are better correlation between the distribution ratio of cracks and the uniaxial compressive strength. (3) The main affecting factors of the deterioration of red sandstones are the transgranular cracks and heterogeneity of cracks.