The invention discloses a temperature measuring device for the wall temperature of a fine rod narrow gap under a motion condition. The temperature measuring device comprises heating element pipes andone or more sheathed thermocouples. One or more temperature measuring holes are formed in the outer circumferential walls of the heating element pipes in the circumferential direction of the heating element pipes. Compensating conductors of the sheathed thermocouples penetrate through the heating element pipes and then extend outwards. The outer walls of the compensating conductors of all the sheathed thermocouples are sequentially wrapped with high-temperature isolating coatings and high-temperature isolating adhesive tapes. The heating element pipes are filled with high-temperature isolatingsilicone. Through the technical scheme, wall surface temperatures of different positions of all the heating element pipes of a high-temperature fine rod bundle narrow gap fuel assembly can be accurately measured under the motion condition, the sheathed thermocouples do not shed or are not subjected to an insulation failure, temperature measuring parts conduct temperature measuring in the rod bundle heating element pipes, thus flow fields and temperature fields beside the heating element pipes cannot be interfered, and thermo-technical performance parameters such as the heat transfer coefficient and the critical heat flux density of the fuel assembly can be accurately obtained by carrying out heat transfer experiments of the fuel assembly under the motion condition.