Abstract: The potassium and silicon nutritional status of crops is closely related to lodging resistance, which is often evaluated by using lignin content. Investigating the regulatory effects of potassium and silicon nutrient coupling on lignin metabolism and lodging resistance of oil flax stems can provide a foundation for lodging resistance through fertilizer management. A three-factor split plot design was used, with variety, potassium dosage, and silicon dosage as the main treatment, split treatment, and sub-split treatment, respectively. In the study, the two varieties were ‘Longya No. 11’ (V1) and ‘Dingya No. 23’ (V2); three potassium dosages were 0 (K0), 52.5 (K1), and 105 kg(K₂O)·hm⁻² (K2); and two silicon dosages were 0 (Si0) and 90 kg(SiO₂)·hm⁻² (Si1). The results showed that there were significant differences in the stem lignin content and related metabolic enzyme activities between V1 and V2. Potassium application significantly increased lignin content, as well as the activities of phenylalanine deaminase (PAL), 4-coumaric acid: CoA ligase (4CL), and cinnamyl alcohol dehydrogenase (CAD); the order of the influence effect was K1>K2>K0. Silicon application did not have a significant effect on these test indicators, but the interaction of potassium and silicon significantly increased the lignin content before the budding stage and improved the activities of PAL, 4CL, and CAD throughout the growth stage, with the best enhancing effect derived from K1Si1 treatment. The lignin content was significantly and positively correlated with CAD activity during the entire growth period, and the increase in CAD activity was an important enzymatic basis for the increase in lignin content. The breaking resistance and lodging resistance index of V1 were higher than those of V2. Compared with K0, K1 significantly increased breaking resistance and lodging resistance index, while K2 had the opposite effect, and the influence effect was K1>K0>K2. Silicon application improved stem breaking resistance and lodging resistance index, and lower potassium accompanied by silicon significantly improved the above-mentioned two indicators. Lignin content was significantly and positively correlated with breaking resistance and lodging resistance index, and negatively correlated with the actual lodging rate. Both lower potassium treatment and lower potassium accompanied by silicon fertilizer significantly increased seed yield. In conclusion, ‘Longya No. 11’ had better lodging resistance than ‘Dingya No. 23’; 52.5 kg(K₂O)·hm⁻² accompanied by 90 kg(SiO₂)·hm⁻² most improved the lignin content, as well as the metabolic enzyme activity, lodging resistance properties, and seed yield; applying potassium fertilizer alone had a better promotion effect on lignin metabolism and lodging resistance than fertilizing silicon individually.