The invention discloses a method for dyeing denim yarn with natural indigo blue. The method comprises the following steps: dyeing denim cotton yarns by using a leuco body multi-time padding oxidationmethod, carrying out vatting reduction at 40-50 DEG C, and dyeing denim yarns according to processes of pre-dyeing treatment, dyeing, oxidation, dyeing, oxidation, dyeing, oxidation, dyeing, oxidation, dyeing, oxidation, dyeing, oxidation and aftertreatmenet with 0.5-2g / L of natural indigo blue, 1.5-4.5g / L of sodium hydroxide, 0.75-1.8g / L of sodium hydrosulfite, 0.25-0.6g / L of a sodium hydrosulfite stabilizer, 10-15g / L of a dyeing acceleration agent sodium chloride or sodium sulfate and 2g / L of a penetrant JFC2. By adopting the dyeing method of multiple times of padding, rolling and oxidation,the dyeing fastness of the denim yarns can be improved, dyeing and oxidation can be both carried out under a room temperature condition, no heating is needed, energy conservation and environment protection can be achieved, reduction is carried out by using a sodium hydrosulfite method, a high reduction velocity can be achieved, and a stable dyeing effect can be achieved.