The invention provides a preparation method of flaky bismuth subnitrate. The preparation method is characterized by comprising the following steps: 1, putting Bi(NO3)3*5H2O into a nitric acid solution, and carrying out ultrasonic dispersion so as to obtain a bismuth nitrate solution; 2, putting the bismuth nitrate solution obtained in the step 1 into deionized water, continuously stirring, heatingto 40-80 DEG C, adding a solution of hexamethylenetetramine 25-35 minutes later, continuously stirring for 2.0-3.0 hours at 40-80 DEG C, stopping heating, further stirring for 1.0-2.0 hours, and leaving to stand for 10-20 minutes, pouring off supernate so as to obtain precipitate; 3, putting deionized water into the obtained precipitate, leaving to stand for 10-20 minutes, and pouring off supernate; 4, repeating the step 3 for 1-3 times, respectively washing for 3-5 times with deionized water and absolute ethyl alcohol, centrifuging, collecting precipitate, dispersing with absolute ethyl alcohol into a crucible, and drying at 60-100 DEG C, thereby obtaining the flaky bismuth subnitrate. The flaky bismuth subnitrate prepared by using the method is good in catalyst morphology and high in catalysis activity, is capable of degrading RhB by 97% or greater within 180 minutes when being used as a photocatalyst, and has good application prospects.