A nanoparticle of glucidamin derived from organism for treating tumor and preparation method thereof, wherein the viscosity-average molecular weight of glucidamin is in the range of 1.times.10.sup.3-9.times.10.sup.5, and the amount of free amino groups in the glucidamin is in the range of 50%-100% based on the total amino groups. The preparation method of the nanoparticle comprises steps listed below: (1) adding the glucidamin possessing one or more molecular weight into thin acid solution at 20-60.degree. C. to form saccharan solution, wherein the content of saccharan in solution is in the range of 0.1%-5% by weight; (2) adjusting the pH of the solution to 6-9 in order to form emulsion of microparticle; (3) separating the microparticle from the emulsion, dried at low temperature to obtain the nanoparticle of glucidamin for treating tumor. The nano-class glucidamin can enhance their activity against tumor. The particle diameter of glucidamin nanoparticle prepared by the method disclosed in the present invention exhibits homogeneous distribution, meanwhile, the nanoparticle is easy to be separated and purified.