Effects of soil temperature variation on photosynthesis of cucumber seedlings were studied in greenhouse by using timed soil warming with heating lines. The results showed that, compared with the control, soil warming at 6:00-8:00 significantly increased soil temperature. Especially at 7:00-12:00, higher soil temperature not only reduced the stomatal limit of cucumber leaves, increased the stomatal conductance and intercellular CO₂ concentration, but also significantly increased Ф_PSII, P_n, AQY and CE of cucumber leaves. When soil was warmed at 12:00, P_n and Ф_PSII of cucumber leaves were 62.3% and 19.67% higher than those of the control, respectively, while soil temperature had little effect on F_v/F_m of cucumber leaves. Soil warming in solar greenhouse significantly promoted the growth of cucumber seedlings by increasing the plant height, stem diameter, and fresh weight; the shoot fresh weight was 113.8% higher than that of the control. In short, soil warming improved the utilization capability of CO₂ and light energy by cucumber seedling leaves, improved photosynthesis of cucumber seedlings and promoted the growth of cucumber seedlings through increasing soil temperature. The results can serve as a theoretical basis for cucumber cultivation in cold seasons.