In order to explore the response mechanism of photosynthesis changes of millet (Setaria italica) seedlings caused by low potassium stress, we used the potassium sensitive cultivar ‘Jingu 21’ and the nonpotassium sensitive cultivar ‘Longgu 25’ as experimental materials. Normal potassium supply (K₅, 5.0 mmol·L^-1), low potassium (K₁, K_0.1, K_0.01, 1, 0.1, 0.01 mmol·L^-1) and without potassium (K₀, 0 mmol·L^-1) were set. We analyzed the effects of different degrees of low potassium stress on the growth of millet seedlings, photosynthetic gas exchange parameters, chlorophyll fluorescence characteristics and activities of key enzymes of carbon assimilation. The results showed that: (1) low potassium stress significantly inhibited the growth of millet seedlings, and the plant height, stem diameter, leaf area and dry matter decreased significantly, while the potassium content was also decreased. Compared with K₅ treatment, leaf while the potassium content of ‘Jingu21’ and ‘Longgu 25’ was significantly decreased by 48.14% and 37.85%, respectively. (2) The photosynthetic pigment content of millet seedlings under K₀ treatment was significantly lower than that under K₅ treatment, but the difference between K_0.1 and K₁ treatment was not significant compared with K₅ treatment. (3) Low potassium stress reduced the net photosynthetic rate (P_n) of millet seedlings, Compared with K₅ treatment, K₀, K_0.01 and K_0.1 treatments significantly decreased P_n, stomatal conductance (G_s), transpiration rate (T_r) and stomatal limit value (L_s), but increased intercellular CO₂ concentration (C_i), demonstrating that the photosynthetic rate reduction is mainly caused by nonstomatal limitation. (4) Low potassium stress can lead to the reduction of photochemical quantum yields of PSⅡ and PSⅠ. Under K₁ treatment, the PSⅡ actual photochemical quantum efficiency (Φ_PSⅡ) and PSⅠ photochemical quantum yield ［Y(Ⅰ)］ of leaves decreased by 5.32%-9.57% and 2.38%-5.63%, respectively. Under K₀ treatment, Φ_PSⅡ and Y(Ⅰ) significantly decreased by 17.15%-20.15% and 18.71%-21.28%, respectively. (5) Compared with K₅ treatment, the activities of ribulose 1,5biphosphate carboxylase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC)of ‘Jingu21’ were significantly decreased by 42.86% and 42.71%, respectively, under K₀ treatment, ‘Longgu 25’ were significantly decreased by 26.85% and 42.77%, respectively. The study found that under the stress of potassium deficiency, ‘Longgu 25’ had stronger absorption and utilization capacity of potassium, and the variation range of each growth physiological parameter was small, showed stronger tolerance to potassium deficiency. Potassium deficiency stress led to the decrease of the net photosynthetic rate of millet seedlings. The decrease of photosynthetic rate under K₀ and K_0.1 treatments was mainly related to the decrease of stomatal conductance, the inhibition of electron transport and energy conversion in the photosystem, and the activities of Rubisco and PEPC decreased, etc.