Abstract: The physiological effects of high temperature, drought and combined stress treatment on Cycas multipinnata were studied by using the leaves of C. multipinnata in vitro. Results show there is no significant change in pigment content after high temperature, drought and dry heat combined stress treatment. The maximum photochemical efficiency and actual photochemical efficiency and electron transfer rate of photosystem Ⅱ are significantly lower than CK. There is no significant difference between high temperature and dry heat combined stress treatment, but they are significantly lower than drought treatment. The quantum yield and photochemical quenching coefficient of non-regulated energy dissipation at the photosystem Ⅱ after drought treatment are not significantly different from the control. The quantum yield of non-regulated energy dissipation at photosystem Ⅱ increase significantly after high temperature and dry heat combined stress treatment, and the photochemical quenching coefficient decrease significantly. Quantum yield of non-regulated energy dissipation at photosystem Ⅱ after dry heat combined stress treatment is significantly higher than high temperature treatment. After high temperature and dry heat combined treatment, the content of malondialdehyde decrease significantly, but its content do not change significantly after drought treatment. The activity of peroxidase is significantly increased after high temperature treatment, and the activities of superoxide dismutase and peroxidase are significantly increased under drought and dry heat stress. After various stress treatments, the soluble sugar content do not change significantly, but the proline content decreases significantly after treatment under high temperature and drought stress. Therefore, the effect of drought on polymorphic cycads is the least, and dry-heat combined stress has the greatest impact on it. Antioxidant enzyme plays an important role in the resistance of polymorphic cycads to high temperature, drought and dry heat.