The invention provides a method for preparing lithium carbonate for batteries by using a low-grade lithium phosphate acidity conversion method. The method comprises the following steps: taking solublesalts of iron, ferrous, aluminum and calcium as converters, enabling the converters to sufficiently react with lithium phosphate under a low-acidity condition, and primarily separating lithium ions from phosphate radicals in the lithium phosphate; adjusting the pH value of a composite decomposition reaction product, and precipitating ferric phosphate, ferrous phosphate, aluminum phosphate or calcium hydrophosphate completely; filtering out the precipitate, putting an alkali substance into the filtrate to adjust the pH value of the filtrate, precipitating ferric ions, ferrous ions, aluminum ions or calcium ions in the filtrate to obtain ferric hydroxide, ferrous hydroxide, aluminum hydroxide or calcium hydroxide, and removing the components to obtain a primary purification liquid; performing ion exchange treatment on the primary purification liquid to obtain a deep purification liquid; enabling the deep purification liquid to react with a sodium carbonate solution at 85-100 DEG C, washing, and drying, thereby obtaining battery-grade lithium carbonate.