The invention discloses an isomorphic synthesis potassium ammonium carbonate mixed crystal compound fertilizer and the production method thereof using the carbonization method to produce ammonium hydrogen carbonate. strong aqua ammonia is added with potassium hydroxide (or potassium carbonate), and then ammonium hydroxide and potassium hydroxide (or potassium carbonate) get carbonized by the carbon dioxide in a carbonization tower, at length the formal name of the potassium ammonium carbonate mixed compound fertilizer comprising X ammonium bicarnate and Y potassium bicarbonate is obtained.