The invention discloses a preparation method of polycrystalline nano cuprous oxide particles, which comprises the following steps: adding a copper source into a 98% polyol solution, and completely dissolving the copper source to obtain a precursor solution; dissolving sodium citrate or glucose in deionized water to prepare a solution, slowly adding the prepared solution into the prepared precursorsolution, and fully stirring until the solutions are uniformly mixed; transferring the prepared mixed solution into a polytetrafluoroethylene reaction liner tube, and putting the polytetrafluoroethylene reaction liner tube into a variable-frequency microwave oven; and centrifugally filtering the reaction product, washing with absolute ethyl alcohol and deionized water, and finally putting the washed sample into a vacuum drying oven to obtain the nano cuprous oxide polycrystalline powder. The method is simple in process, easy and convenient to operate, short in reaction time, low in cost consumption and environmentally friendly in raw materials, and the prepared cuprous oxide nanoparticles are of a polycrystalline structure, narrow in particle size distribution and good in stability.