The invention discloses a continuous determination method of a potassium nitrate and sodium nitrate mixed salt bath. The method provided by the invention comprises the following steps: preparing a sample solution, determining and calculating the content of potassium nitrate, determining and calculating sodium nitrate and the like. In the acidic medium of acetic acid, sodium tetraphenylborate reacts with potassium nitrate to produce a potassium tetraphenylborate precipitate, filtering, drying and weighing are carried out, and then the content of potassium nitrate is calculated; in the presence of molybdenum salt, a concentrated sulfuric acid is used to acidify, the nitrate reacts with excess ammonium ferrous sulfate standard solution, sodium diphenylamine sulfonate is used as an indicator, a potassium dichromate standard titration solution is used for back titration, the total amount of the nitrate is calculated according to the amounts of the ferrous ammonium sulfate and the potassium dichromate standard titration solution, and the amount of potassium nitrate equivalent to sodium nitrate is subtracted to obtain the content of sodium nitrate. The determination method provided by the invention has the advantages of simplicity, convenient detection process, quickness, environmental protection, low cost, and accurate and consistent detection result, and provides support for the maintenance and quality control of potassium nitrate and sodium nitrate.