To reveal mechanism of influence of environmental factors such as ambient temperature, sunshine and wind velocity on infrared thermographic nondestructive testing, modeling of a delamination in a glass fiber reinforced plastic specimen in infrared flash thermography testing was made and studied. The model was an axial symmetry model. Thermal signals to be detected in infrared thermographic nondestructive testing, such as temperature difference, the maximum temperature difference, temperature contrast and the maximum temperature contrast, were calculated and analyzed using finite element method in that model. Several experiments with special process treatment on that specimen were carried out. Evolution of the thermal signals with changes of those environmental factors was researched. Comparison of results from modeling and experiment in ambient temperature 25 ℃ and 30 ℃ was made. Results from modeling and experiments in uneven sunshine and in homogeneous temperature were compared too. Results in deferent wind speed were presented at the end of this paper. Convective heat transfer coefficients in the model were 10 W/(m²·K) and 100 W/(m²·K). The simulation results and the experiment results show that the maximum temperature contrast declines with increase of ambient temperature. Decline of the maximum temperature contrast means that clarity of defect gets worse, and detection of defect becomes more difficult. The maximum temperature difference and the maximum temperature contrast increase or decrease due to uneven sunshine on the specimen. Those changes lead to misjudgment or missing detection. Increase of wind velocity may reduce the maximum temperature difference and the maximum temperature contrast, and result in worse detectability and clarity of defect.