What is disclosed in the invention is a preparation method of a supercritical Cinnamomum subavenium extract, which is made from the material, the dried stem of C. subavenium. The extract is obtained by extracting C. subavenium which is pulverized as particles with supercritical carbon dioxide fluid. The C. subavenium extract or its active ingredient, subamolide A, can be used to inhibit the growth of human urothelial carcinoma cell lines. In addition, the C. subavenium extract (or subamolide A) is able to synergistically inhibit the growth of human urothelial carcinoma cell lines with cisplatin (CDDP) or gemcitabine (Gem). Therefore, the C. subavenium extract (or subamolide A) can be an anticancer drug alone, or forms a pharmaceutical composition with CDDP (or Gem) to treat with cancers in respect of urinary system.