This paper gives out a brief summarize of the reason and hazard of aircraft icing and introduce the means of windshield anti-ice firstly, then analyze the anti-ice heat losses of flat windshield which has been widely-used in commercial aircrafts and illuminate the purpose and calculating method of every portion of heat losses. Selection principle of state point for heat load calculation has been determined according to calculation input. In addition, inward heat loss to the cockpit when windshield works has been taken into consideration. The paper presents a method of anti-ice heat load for windshield and calculation result is close to heat power of windshield for several aircrafts. The presented method has been discussed with representatives of CAAC and been approved, and could be used in engineer design.