Photosynthetic rate is reduced during midday in some crops; this phenomenon has been termed as midday depression (MD). Oncidium also suffers greatly from MD in the summer, resulting in reduced growth and poor flowering quality. Since high radiation usually accompanies high temperature midday in the summer, it is difficult to figure out the key factor that promotes MD. We investigated the photosynthetic activities of Oncidium Gower Ramsey in the following conditions: environment-controlled and nonenvironment-controlled. In a growth chamber that simulated field growth conditions, photosynthesis declined dramatically when the temperature was higher than 32 °C. Photosynthesis was also reduced when photosynthetically active radiation (PAR) exceeded the saturating point of Oncidium. Gower Ramsey, which is about 250 μmol·m-2·s-1. However, the reduction was slight when PAR was under 500 μmol·m-2·s-1. Daily photosynthetic patterns were changed when Oncidium Gower Ramsey was grown under different environments. By regression, we found that MD was not directly associated with PAR within the range of 0–400 μmol·m-2·s-1. By contrast, photosynthesis was significantly reduced when temperature was higher than 32 °C. This explains the observation of greater photosynthetic reduction and earlier occurrence of MD when Oncidium Gower Ramsey was grown in rain-shelter rather than in phytotron and growth chamber, since temperature in the rain-shelter was not controlled, while the others were controlled at 25 °C. When Oncidium Gower Ramsey was moved from 35 °C to 25 °C, the photosynthetic depression was relieved.