The invention provides a preparation method of W18O49 nanoparticles having both photothermal therapy and CT (Computerized Tomography) radiography functions. The preparation method comprises the following steps of: (1) weighing WC16 and dissolving the WC16 in a reaction solvent under the condition of nitrogen protection; (2) adding a polyacrylic polymer to the solution formed in the step (1), reacting for 0.5-8h while heating and stirring under the nitrogen protection condition, and then reducing the temperature to the range from 60 to 100 DEG C, next, continuing adding a washing solvent, continuing stirring and reacting for 0.5-4h, centrifuging and purifying the reacted solution and abandoning the supernate; and (3) repeatedly washing and dispersing the product obtained in the step (2) by using de-ionized water to obtain purified W18O49 nanoparticles, wherein the size of the W18O49 nanoparticles ranges from 3 to 15nm, and the hydration particle size of the W18O49 nanoparticles ranges from 8nm to 45nm. The method provided by the invention is simple in process; the prepared nanoparticles are capable of realizing attenuation and absorption to X-rays, and also have a surface plasma resonance absorption capability similarly to a noble metal; and under laser excitation, the W18049 nanoparticles are capable of converting the energy of laser into heat to kill cancer pathological cells.