An experimental research is conducted on the heat transfer characteristics of a loop heat pipe (LHP) used in the “mainline” heat transfer mode for spacecraft platform thermal control. The heat from multiple instruments scattered in different locations is collected by thermal control techniques such as axially grooved heat pipes and then transmitted to the radiant surface for dissipation through the LHP in an unified way. The research contents include the start-up characteristics, the operational stability characteristics, the operational blocking characteristics, the continuous blocking characteristics, the heat transfer capability, the thermal resistance, and the dynamic response characteristics under the change of the heat sink temperature. The results show that the higher the auxiliary starting power is, the easier it is to start the LHP; the higher the input power of the thermoelectric cooler is, the more beneficial it is to speed up the stabilization of the vapor-liquid interface in the condenser; the higher the blocking power, the shorter the blocking time of the LHP; the LHP can be operated stably within the heat sink temperature alteration process; the heat transfer ability is higher than 500 W with a systematic thermal resistance of 0.037°C/W.