18β-Glycyrrhetinic acid (18β-GA) is the main active component of licorice and one of the most promising lead compounds in traditional Chinese herbal medicine. Previous studies have shown that 18β-GA can act as a PPARγ agonist to exert antitumor activity. However, the number of reported 18β-GA derivatives as PPARγ agonists is limited and they have significant toxic side effects, which greatly restricts their application and development. To obtain highly effective and low-toxic PPARγ agonists, through structure-activity relationship (SAR) analysis, we constructed a molecular library of 18β-GA derivatives containing 13,440 compounds and screened out 14 novel 18β-GA ester derivatives. The selected compounds were evaluated for their antitumor activity in vitro. The results showed that most of the compounds exhibited strong anti-proliferative activity against five human cancer cell lines, especially the human colon cancer cell line HT-29, without significant toxicity to normal cell lines. Among them, C1 had the strongest anti-proliferative activity against HT-29, with an IC