The deformation and stress condition of exposed steel penstock under non-uniform temperature due to sunshine during overhaul are analyzed through ANSYS which is based on a typical part of penstock in a hydropower station. Further simulations on the deformed penstock applied with internal water pressure are also carried out. Results indicate that when bearing internal water pressure, the deformed exposed penstock induced from the former non-uniform temperature shows obvious increasing lateral displacement to the cold side and bending in horizontal plane. The temperature effects are more obvious if the pressure is higher. A bigger gap between supports and lateral dampers brings about lower reaction around supports and constraint plates while weakens the restraint of dampers to penstock. In order to strengthen the restriction to penstock and relieve reaction, it is recommended to reduce the distance between pier foundations around bellows expansion joint because deformation and reaction here are enormous. Meanwhile, enlarging the lateral damper appropriately is necessary to avoid penstock sliding away from supports under the non-uniform temperature.