Both abiotic and biological stresses reduce the quality and quantity of cultivated plants. In order to observe the response of cucumber plants to potassium stress and cucumber downy mildew infestation, two different levels of downy mildew infestation, B0 (disease infestation) and B1 (disease-free infestation), and three fertilizer requirement levels of potassium fertilization, T1 (K-50%), T2 (K-100%) and T3 (K-150%), were applied in the greenhouse. Results show that the photosynthetic rate and stomatal conductance of cucumber plants leaves treated with B1T2 had an increasing trend, and the increase in stomatal conductance was more significant. The intercellular CO₂ concentration of cucumber leaves treated with B1T2 showed no significant difference. The plant height of cucumbers treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 32.21%, 16.93%, 21.59%, 53.54% and 6.31%, respectively, compared with that treated with B1T2. The leaf area of cucumbers treated with B0T1, B0T2, B0T3, B1T1 and B1T3 decreased by 27.85%, 15.97%, 21.47%, 7.29% and 18.5%, respectively, compared with that treated with B1T2. The stem diameter of cucumbers treated with B0T1, B0T2, B0T3 and B1T1 decreased by 13.43%, 8.72%, 6.01% and 7.06%, respectively, compared with that treated with B1T2. The stem diameter of cucumbers treated with B1T3 increased by 6.83% compared with that treated with B1T2. The root total length, root surface area, root average diameter, root total volume and total root tips of cucumber plants were significantly different under different experimental conditions (p < 0.05). This study provides a theoretical basis for cucumber stress resistance cultivation in greenhouses and has important practical significance to ensure the sustainable development of the greenhouse cucumber industry.