Abstract: The present invention discloses the application of an ursodeoxycholic acid and its pharmaceutical salt in the preparation of an antitumor drug and an antitumor drug. Through research, it is discovered that ursodeoxycholic acid and its pharmaceutical salt have good inhibition effect on a variety of tumor cells, and further research reveals that they function by reducing the proportion of regulatory T cells in tumor infiltrating lymphocytes. Therefore, ursodeoxycholic acid can be used in the preparation of an antitumor drug. Besides, as a clinically used choleretic drug, ursodeoxycholic acid does not require clinical safety evaluation, and has good application prospects. It can also be used as an ancillary drug to improve the therapeutic effect of anti-PD-1 drugs.