In order to provide theoretical basis and effective way to alleviate the continuous cropping obstacle of cucumber, we studied the effects of different concentrations of exogenous silicon (0, 0.5, 1.0, 1.5 and 2.0 mmol/L) on the growth, photosynthesis and chlorophyll fluorescence parameters of ‘Lübo 6’ cucumber seedlings under autotoxicity (3.0 mmol/L cinnamic acid, CA) by nutrient solution culture method. Our results indicated that: (1) autotoxic stress significantly inhibited the growth of cucumber seedlings, root morphogenesis and biomass accumulation, significantly reduced the net photosynthetic rate (P_n), transpiration rate (T_r), stomatal conductance (G_s) and chlorophyll (Chl a, Chl b and Chl t) contents. At the same time, PSⅡ electron transport rate (ETR), PSⅡ actual photochemical efficiency ［Y_(Ⅱ)］ and light chemical quenching coefficient (qP) of cucumber leavers were markedly decreased. (2) Adding appropriate concentration of exogenous silicon can effectively alleviate the effect of CA stress on the growth of cucumber seedlings, improve the values of P_n, G_s, T_r and chlorophyll contents, so as to maintain the stability of the photosynthetic system of leaves to a certain extent. (3) Under CA stress, adding suitable concentration of exogenous silicon significantly increased F_v/F_m, ETR, Y_(Ⅱ), and qP in leaves of cucumber seedlings, rather than the coefficient of photochemical quenching (NPQ) dropped significantly. The results showed that the contents of chlorophyll, F_v/F_m, ETR, Y_(Ⅱ) and qP of cucumber seedlings under CA stress were increased by adding appropriate concentration of exogenous silicon, which made the photosynthetic apparatus stable, inhibited the decline of P_n, and alleviated the damage of autotoxicity on the photosynthetic system, so as to improve the resistance of cucumber seedlings to CA stress. The optimal concentration of exogenous silicon was 1.0 mmol/L.