The invention relates to a passive targeting antitumor prodrug of solid tumor and a preparation method thereof, belonging to the field of antitumor drugs. The preparation method comprises the steps of: reacting small molecules of pectin with Mw of 5,000-45,0000 with doxorubicin to obtain pectin and doxorubicin conjugate with Mw of 100,000-1,000,0000, preparing a suspension, and processing the suspension by using a nano ultra high pressure homogenizing machine to obtain the passive targeting antitumor prodrug of solid tumor with particle diameter of 100-200 nm and melting point of 220-245 DEG C, wherein the pectin and the doxorubicin are connected by an amide bond, the pectin and the pectin are connected by an ester bond formed by condensing the carboxyl group and the hydroxyl group of pectin molecules. The cell inhibition rate of the antitumor prodrug to humanized lung carcinomas cells NCI-H446 and A549 is equivalent to that of the doxorubicin hydrochloride. In the research of the curative effect of melanoma B16 pulmonary metastasis mode mouse, the life cycle of mouse-borne tumor is 42.3+ / -12.4 days, which is remarkably higher than 23.1+ / -10.2 days of the doxorubicin hydrochloride group.