Abstract: In order to study the pore structure change characteristics of basalt fiber reinforced concrete under two sources of damage, high temperature and mechanics, nuclear magnetic resonance(NMR) and scanning electron microscopy(SEM) techniques were used to observe the T₂ spectrum distribution, pore size distribution and flaw development of the basalt fiber reinforced concrete. The results show that normal concrete, short basalt fiber reinforced concrete and long basalt fiber reinforced concrete show a trend of decreasing the number of micropores and increasing the number of mesopores after high temperature. By comparison, it is found that long basalt fiber reinforced concrete has the largest number of pores in the main peak of the T₂ spectrum and the largest pore size distribution. The long basalt fiber reinforced concrete was taken as an example to illustrate the change characteristics of pore structure under two kinds of damage sources. The number of pores under high temperature damage with a relaxation time of 0.1–10 ms is greater than the number of pores under mechanical damage. And the main peak of T₂ spectrum shiftes to the right as the temperature increases. However, the main peak of T₂ spectrum hardly changes with the increase of load, indicating that the increase in temperature can aggravate the damage more than mechanical. And the new pore diameter increases continuously under each temperature. The main peak of T₂ spectrum and pore size distribution increase with temperature increasing, and decrease at first and then increase with increasing load, which indicates that the high temperature directly causes damages to the concrete, while the mechanical makes the concrete dense at first and then damage. The same conclusion is obtained by electron microscope observation.