The invention relates to a nanocomposite temperature-sensitive gel and a preparation method and an application thereof. The preparation method comprises the following steps of: coating antitumor active substances with a high molecular polymer serving as a carrier material to obtain nanoparticles; and adding a temperature-sensitive high molecular material to obtain the nanocomposite temperature-sensitive gel. The preparation method disclosed by the invention is simple and convenient, is suitable for large-scale production, and is particularly suitable for preparing medicaments or diagnostic reagents having the characteristics of long cycle, biodegradability, slow release, passive targeting, active targeting, active substance conveying function and tumor resistance. An antitumor medicament prepared with the method disclosed by the invention is suitable for ways such as intravenous injection, intramuscular injection, hypodermic injection, intradermal injection, intratumor injection, tumor-side injection, oral administration or transdermal medicament delivery and the like, is applied to treatment and diagnosis of pancreatic cancer, liver cancer, lung cancer, gastric cancer, colorectal cancer, esophageal cancer, prostatic cancer, uterine cancer and ovarian cancer, and has a good application prospect.