The invention relates to a Cr/CrNx heat insulating film for the top surface of an aluminium alloy piston and a preparation method of the Cr/CrNx heat insulating film. The Cr/CrNx heat insulating filmis formed by a pure metal Cr layer attached to the top surface of the aluminium alloy piston and a CrNx layer coating the pure metal Cr layer uniformly. The preparation method of the Cr/CrNx heat insulating film comprises the following steps: firstly, the cleaned and dried aluminium alloy piston is placed in an FCVA (filtered cathodic vacuum arc) deposition system, a magnetic filtered Ti power arcis switched on, and the pure metal Cr layer is deposited with a pure metal Cr target as a cathode; the aluminium alloy piston with the pure metal Cr layer on the top surface is obtained; then the pure metal Cr target is taken as the cathode, nitrogen is taken as working gas, the nitrogen introducing amount is regulated, and the CrNx layer is generated on the pure metal Cr layer of the aluminium alloy piston with the pure metal Cr layer on the top surface. The strength, the wear resistance and the heat insulation performance of the obtained product are all better than those of the existing products, requirements of modern engines for high power and low emission can be met, and large-scale industrial application is facilitated.