Abstract: To investigate the synergistic effect of polyaspartic acid (PASP) applying under the condition of potassium fertilizer reduction, by taking potassium applying at a conventional application rate as the control (CK2), the effects of potassium reduction combined with supplement of polyaspartic acid on tobacco-planting soil and potassium absorption and utilization of flue-cured tobacco were studied at four PASP application rates, namely 0 (KP0), 2.5% (KP1), 5% (KP2) and 7.5% (KP3) while reducing potassium fertilizer by 10%. The results showed that, compared with CK2, soil nutrient contents (mass fractions), soil enzyme activity and potassium absorption and utilization rate in the treatment KP0 decreased significantly. Compared with KP0, the nitrogen and potassium contents and enzyme activities of soil in the treatments KP1, KP2 and KP3 were promoted to different extents. In the treatment KP2, the available potassium content in soil increased by 192.44% and the activities of acid phosphatase and urease increased by 20.04% and 24.74% respectively on the 50th day after the supplement application of potassium fertilizer. The potassium absorption amount of tobacco plant was the largest in the treatment KP2, which was 96.52 kg·hm^-2, higher than the treatment KP0. Soil potassium surplus in the treatment KP2 decreased by 60.30% compared with the treatment KP0. The utilization rate of potassium fertilizer combined with PASP increased greatly and was significantly higher than that of CK2. Compared with the treatment KP0, the agronomic efficiency, partial productivity and current seasonal recycling rate of potassium in the treatment KP2 raised by 30.19%, 16.64% and 82.98% respectively. The potassium content in tobacco leaves was higher in the treatment KP1. In conclusion, potassium reduction combined with PASP application can promote the nutrient content and enzyme activity of tobacco-planting soil, decrease soil potassium surplus, improve the potassium absorption and utilization of flue-cured tobacco and raise the yield and potassium content of cured leaves. Under the condition of this experiment, the effect of supplement application with 5% PASP was better.