The invention discloses a cobalt selenide nano material and an application thereof. A preparation method for the cobalt selenide nano material comprises the following steps: adding a selenium-containing compound and a cobalt salt into a solvent, and fiercely stirring at a room temperature until uniformly mixing; then, pouring a mixed solution into a reaction kettle, and heating for reacting at 180 DEG C for 10-20 hours to obtain precipitates; centrifuging and washing the obtained precipitates, and adding the centrifuged and washed precipitates into an aqueous solution containing a stabilizer; and performing ultrasonic treatment and stirring to obtain a stabilizer-modified water-soluble cobalt selenide nano material. The preparation method for the cobalt selenide nano material disclosed by the invention is simple and convenient to operate, low in energy consumption and easy for large-scale production. The prepared nano particle is uniform in particle granularity and good in dispersion, can be used as a contrast agent for photoacoustic imaging and magnetic resonance imaging, and has important application prospect in the tumor imaging field.