This paper focuses on the thermal barrier effect of magnesium-fluoride conformal dome. Firstly, the real flight process was simulated and a finite element analysis model of the fairing aerodynamic heating and internal heat transfer were established under different flight speeds, the aerodynamic-thermal loads and the average temperature of dome was calculated. Secondly, according to the basic principles of infrared detection, each temperature dome heat radiation and its effects on the imaging quality of the detection system were analyzed, the critical temperature of dome was calculated when detector reached the saturation condition to study the mechanism of the thermal barrier effect. Finally, imaging tests were carried out for validation of analytical results. The results show that the critical temperature of dome when detector reaches the saturation condition is about 460 K, the thermal barrier effect is remarkable under the condition of Ma=4 and final guidance in Ma=3. In order toaccelerate the infrared imaging guided weapons flight speed, the thermal barrier effect must be corrected. The conclusions has certain reference for understanding the mechanism of the thermal barrier effect and correction methods.