The invention discloses a preparation method of a nano film ammonia gas sensor. The preparation method comprises the following steps: weighing aniline monomers (5-500 mol/L), dissolving aniline monomers in a sulfuric acid solution (0.1-1 mol/L), then adding nickel (II) phthalocyanine tetrasulfonic acid tetrasodium salt (0.1-10 mol/L), stirring to evenly mix the components to obtain electrolyte (A); carrying out cyclic voltammetry on the electrolyte (A) to precipitate a polyaniline-nickel (II) phthalocyanine tetrasulfonic acid composite film on an interdigital gold electrode, washing the interdigital gold electrode for 2 to 3 times, and drying the interdigital gold electrode by blowing nitrogen gas to the interdigital gold electrode so as to obtain the nano film ammonia gas sensor. The preparation method is simple, the respond time is short, the selectivity is good, the sensitivity is high, moreover, the stability and repeatability are both very good, and the prepared sensor can work at a room temperature and can be used in the fields such as environment monitoring, electronic nose, disease diagnosis, and the like.