The invention discloses a method for preparing polyphosphoric acid and ammonium polyphosphate by conducting hydrochloric acid decomposition on medium-low-grade phosphorus ores. Any medium-low-grade phosphorus ores can be decomposed by adopting hydrochloric acid, decomposition liquor is neutralized by using lime milk, then extraction is performed with TBP, reverse extraction is performed with pure water, concentration and polymerization are performed to prepare polyphosphoric acid, and ammonia is introduced to the polyphosphoric acid to polymerize the ammonium polyphosphate. An extraction raffinate water phase and a whiter fertilizer produced in neutralization are pulped for redissolution by adding the hydrochloric acid, heating is performed to repel out HF, chloride salts out of the solution, filtration washing is performed, a salting-out mother solution and washing water are returned for phosphorite decomposition, the chloride is dissolved with water, the lime milk is added for neutralization, filtration washing is performed, and filtrate washing water is concentrated and crystallized to generate anhydrous calcium chloride; a wet solid phase is dissolved with hydrochloric acid, iron is extracted with N235, the ammonia is absorbed to generate ferric ammonium salt, air is introduced to the ferric ammonium salt, and iron oxide red is prepared through heating. Ammonia is introduced to the extraction raffinate water phase to produce magnesium hydroxide, and boron-free magnesium oxide is prepared through filtration washing and calcination. The filtrate obtained after filtration is thermally decomposed to produce aluminum oxide powder, and a gas phase is cooled to generate ammonium chloride. The ammonium chloride and silicon residues are calcinated at medium temperature to generate a high-activity silicon fertilizer and hydrogen chloride, and the hydrogen chloride is returned for white fertilizer decomposition.