The invention discloses a preparation method of an Ag/Cu2O heterojunction nano film. A pure Cu sheet and a Cu(NO3)2 solution with a concentration of 0.01-0.10 mol/L are made to react at 80 DEG C-90 DEG C for 4-6 hours, and a Cu2O film with a photovoltage value of 0.0937 V-0.3168 V is prepared on the Cu sheet; a Cu(NO3)2 solution with a concentration of 0.05-0.09 mol/L and an AgNO3 solution with aconcentration of 0.0005-0.0025 mol/L are added into the Cu2O film, and reaction is conducted in an oven for 2-8 hours to obtain the Ag/Cu2O heterojunction nano film with a photovoltage value of 0.3273V-0.3853 V. The preparation method has simple process, short cycle and low requirements on raw materials and equipment, and the obtained heterojunction nano film is more favorable for being excited by sunlight to generate light-generated electrons and has excellent photoelectric performance.