Abstract: The pneumatic control wing rudder surface of a high-speed aircraft has a gap between the projectile and the wing rudder surface for flexibility. The existence of the gap leads to the entry of high-speed hot airflow as well as the strong separation and reattachment area at the root of the rudder shaft with high heat, high pressure, and high shear thermal environment. These severe environments require high thermal protection of the aircraft. Since the factors affecting the flow of the rudder gap are very complicated, accurate prediction of the thermal environment inside the gap is very difficult. At present, the traditional shock wave test of the wind tunnel is limited by the 2 mm diameter of the thin film heat flow sensor. Only limited measurement points can be arranged in the separation and reattachment area, and the peak of the heat flow cannot be captured, resulting in large differences between the calculation and the test. According to the characteristics of the thermal environment of the gap separation and re-attachment area, this paper analyzes the feasibility of fine measurement regarding sensor selection, measurement point layout scheme, measurement, and data post-processing. Moreover, a distributed thermocouple fine measurement method is proposed to realize the use of spot heat measurement for the effect of surface heat. Aiming at the simplified model of the cylindrical body and the rudder surface, the fine thermal test of the rudder gap is completed, and the peak heat flow in the rudder interference zone is obtained. The effects of different gap heights, rudder angles, and angles of attack on the thermal environment of the rudder interference zone are studied experimentally. The experimental results show that the rudder gap interference on the missile body is mainly concentrated in the rudder axle interference zone. The thermal environment of the rudder axis interference zone increases with increasing gap height, rudder angle, and angle of attack. The comparison between the test results and the CFD calculation results shows that they are basically consistent, and the peak heat flow difference is within 10%.