To indicate the capability of windshield anti-ice is adequate under all combinational cases of flight envelope and icing condition,the design requirement and flow field characteristics of windshield is analyzed ,a theoretical conservative engineering model of windshield anti-ice thermal analysis is established. Key coefficient of the model is corrected and verified by flight test data. The model can be implemented by general programming language or EXECL internal function to deal with thermal analysis under all combinational cases. The results could support airworthiness certification or filter CFD cases.