Process for preparing indigo and its intermediate

An indigo is prepared from anilinoacetonitrile as main raw material, anhydrous K-Na alkali mixture as solvent, and aminosodium as condensating agent through adding potassium sodium (or potassium) anilinoacetate, cyclyzing the potassium sodium anilinoacetate to become the indolol potassium (or sodium) at high temp and pressure, oxidizing, press filtering and drying. Said potassium sodium (or potassium) anilinoacetate as intermediate is prepared through proportionally mixing anilinoacetonitrile with the liquid K-Na alkali mixture or liquid K alkali.