The invention relates to a method for determining manganese in low alloy steel by using a colorimetric method. The method comprises the following steps: weighing appropriate samples and putting in a 100ml steel volumetric flask; adding 10ml of nitric acid (of which the concentration is 33.3 percent), heating for dissolving (synchronously shaking and heating, thoroughly exhausting nitric acid smoke), slightly boiling, and taking down; adding 5ml of phosphoric acid (of which the concentration is 85.11 percent), adding 10ml of periodate potassium (sodium) water solution (of which the mass fraction is 5 percent), heating and boiling for 20-30s, and taking down; adding water for diluting graduation, and shaking up, at the position with a wavelength being 530-550nm, in a 1-3cm cuvette, using water as a reference, measuring the absorbance, and using a standard sample evaporator. The method does not use perchloric acid or nickel chromium, is small in pollution, does not use highly toxic arsenious acid, and is short in analysis time, quick and stable and accurate in results.