In order to effectively reduce the collapsibility of loess areas in contact with water, a study was conducted on the collapsibility of loess using microbial induced calcium carbonate precipitation (MICP) in Lanzhou loess. The unconfined compressive strength and coefficient of collapsibility of the loess before and after MICP treatment were evaluated. The microscopic characteristics of loess modified by MICP were analysed by CT scanning technology, and the improvement mechanism of MICP was further explored. The test results show that the unconfined compressive strength of the loess specimens modified by MICP increases significantly, up to 150 kPa, and the collapsibility of the loess is effectively reduced; the amount of calcium carbonate production continues to increase with the increasing concentration of the cementing fluid, but decreases when the concentration of the cementing fluid is greater than 1.25 mol/L; the porosity and pore equivalent diameter of the modified loess specimens are decreased. The porosity and pore equivalent diameter of the modified loess specimens are all reduced.