This paper solves the problem that is met during civil aircraft windshield de-fog performance certification phase. As the environment defined in §25. 773(c) is too general to conduct windshield de-fog flight tests, a high temperature with high humidity(19g water/ kg dry air, ambient temperature 40℃) is introduced as the critical case for windshield de-fog flight test after analyzing the principle of fogging on aircraft windshield. The paper proposes two different methods to define the de-fog flight test environment, hold humidity while drop temperature, or hold guaranteed class for system designing, as the dedicated high temperature with high humidity is hardly to meet in ambient air. Applicable curves for flight test are plotted on standard atmosphere enthalpy and humid chart, relative humidity-environment temperature chart respectively. The curves have been proved feasibly by other type aircrafts de-fog flight test, and the curves could be used to find an appropriate environment for civil aircraft windshield de-fog flight test. Flight test method for windshield de-fog is also discussed in this thesis.