Abstract: The hardness distribution of the friction stir welded joint of 6061-T6 aluminum alloy under the stirring needle rotation speed of 960–1 800 r · min⁻¹ was studied. The finite element simulation of temperature distribution during friction stir welding was carried out by ABAQUS software, and was verified by the temperature change of the temperature measurement points in the heat affected zone. The influence of stirring needle rotation speed on the welding temperature in the heat affected zone was simulated and studied, and the corresponding relationship between the welding temperature distribution and the joint hardness distribution was established. The results show that the lowest hardness of the joint was located in the heat affected zone or the welding core + thermo-mechanically affected zone. When the rotation speed of the stirring needle was not higher than 1 200 r · min⁻¹, the hardness of retreating side law hardness (RLH) region was lower than that of advancing side low hardness (ALH) region, and RLH region was the weakest position of the mechanical properties of the joint. When the rotation speed of the stirring needle was higher than 1 200 r · min⁻¹, the hardness of ALH region was lower than that of RLH region, and ALH region was the weakest position. The temperature variation trend of different temperature measurement points in the heat affected zone was consistent with the test results, and the maximum relative error of the peak temperature was 2.6%, indicating the model could accurately predict the temperature distribution in friction stir welding 6061-T6 aluminum alloy. When the rotation speed of the stirring needle was no more than 1 200 r · min⁻¹, the peak temperature in RLH region was higher than that in ALH region, resulting in more precipitates dissolving and lower hardness. When the rotation speed of the stirring needle was higher than 1 200 r · min⁻¹, the peak temperature in ALH region was higher, the residence time in the high temperature region was longer, the precipitates dissolved more seriously, and the hardness was lower. The region where the peak temperature exceeded 400 ℃ and the cooling rate was the slowest obtained by simulation was the weakest position of the mechanical properties of the joint.