Abstract: To explore the effects of potassium supply on the lodging resistance of oil flax, split-plot experiments were conducted with the cultivars 'Longya No. 11' (V1) and 'Dingya No. 23' (V2) and four potassium (K₂O) levels: no potassiumK0, 0 kg(K₂O)·hm^-2, low potassiumK1, 30 kg(K₂O)·hm^-2, medium potassiumK2, 60 kg(K₂O)·hm^-2, and high potassiumK3, 90 kg(K₂O)·hm^-2. The lignin content, lignin synthesis-related enzyme activities, stem mechanical properties, lodging resistance index of the stem, and yield of oil flax under different potassium supply levels were analyzed. The results showed that under different potassium levels, the relationships between the plant physical characteristics and lodging were mainly observed from the green fruit stage to the maturation stage. The lodging resistance index of oil flax was significantly positively correlated with culm diameter and negatively correlated with plant height and gravity center height. The lignin content of the oil flax stems changed during the growth period and with the potassium supply. The lignin content of V1 and V2 was higher in K2 treatment after potassium application from the seedling stage to the flower stage. Compared with that under K0 treatment, the lignin content of V1 and V2 under K3 treatment increased by 17.68% and 23.25%, respectively, from the green fruit stage to the maturation stage. The effects of cultivar, potassium application, and their interactions on the lignin content at the green fruit stage reached extreme significance (P < 0.01). The K2 treatment increased the phenylalanine ammonia-lyase (PAL) activity of the stem from the budding stage to the green fruit stage and the tyrosine ammonia-lyase (TAL) activity of the V1 cultivar at the budding stage; the K3 treatment increased the TAL activity of the V2 cultivar. Potassium fertilizer significantly affected the cinnamon alcohol dehydrogenase (CAD) activity of the oil flax stem at the green fruit stage. Compared with that in the no potassium treatment, the CAD activity of two cultivars increased by 31.96% and 12.09% on average, respectively, after potassium application. K2 and K1 are the suitable potassium supply levels for peroxidase activity improvement in the V1 and V2 cultivars, respectively. Compared with the indexes at K0, the snapping resistance and lodging resistance indexes of V1 in the K2 and K3 treatments increased by 45.68% and 48.90%, respectively, and by 16.86% and 31.92%, respectively; V2 snapping resistance increased by 84.32% (K2) and 77.50% (K3), respectively, and the lodging resistance index increased by 1.89% (K2) and 14.49% (K3), respectively. Correlation analysis showed that the four lignin related enzymes were positively correlated with lignin content and lodging resistance index at the green fruit stage. After potassium application, the highest grain yield was observed in the K3 treatment, which increased by 10.71% for V1 and by 17.77% for V2 compared with that in the K0 treatment. There was no significant difference in the yield of V1 among different potassium levels. However, the yields of V2 in the K2 and K3 treatments were significantly higher by 12.24% and 17.77%, respectively compared with that of K0. These results indicate that the cultivar is an important influencing factor of the lignin accumulation process of oil flax and that potassium fertilizer has a synergistic promotional effect on the lignin content and the metabolism-related enzyme activities. In this experiment site and the farmlands with similar ecological environments, the use of medium and high potassium fertilizers60-90 kg(K₂O)·hm^-2 promotes the synthesis and accumulation of oil flax stem lignin, prevents oil flax lodging, and establishes a foundation for a high and stable yield.