In order to investigate the hardness variation of slim acetate fiber filter rod (hereinafter referred to as slim filter rod) and improve the quality of slim filter rod, the influencing factors of the dosage of glycerol triacetate, the temperature of glycerol triacetate, the natural curing time of slim filter rod, the filling amount of acetate tow, the permeability of plug wrap paper, the circumference of slim filter rod and the pressure drop on the hardness of slim filter rod were studied. The results showed that the effect of the dosage of glycerol triacetate on the hardness of the slim filter rod and the filter rod was the same under the condition of natural curing for 1 day. The hardness of the slim filter rod increased with the increase of the dosage of glycerol triacetate in the range of 5%～23% under the control of the conditions of natural curing for 2～5 days. Within 5 days of natural curing, the hardness of the slim filter rod increased rapidly, and slowed down within 5～30 days. There was no correlation between the temperature of glycerol triacetate and the mean hardness and the stability of the slim filter rod. The hardness of the slim filter rod increased with the filling amount of acetate tow. The hardness of the slim filter rod increased with the increase of the permeability of plug wrap paper. The circumference of the slim filter rod was negatively correlated with the hardness. The pressure drop of the slim filter rod was correlated with the hardness.