Space structures encounter various severe environments in space. One of these environments is severe thermal condition where the difference of temperature during day-time and night-time is about 200 degrees Celsius. A signal level of radio wave from the Large Deployable Reflector (LDR) mounted on the Engineering Test Satellite-VIII (ETS-VIII) was observed to change during the Earth eclipse. This phenomenon was assumed to be caused by thermal deformation of the LDR. It was not a critical issue for the ETS-VIII because the communication beam from the LDR tended to spread in the wide range. However, highly accurate pinpoint communication beams are expected to be required in the future, and this transition of the beams may affect the performance of such satellites. Therefore, we tried to seek out the means to suppress the thermal deformation mechanically by focusing into the internal force generated at springs used to deploy the antenna. Furthermore, we simulated the thermal deformation behaviour in the original model of the antenna. According to the analyses in the single module model, the deformation at central side of upper radial member is almost suppressed by controlling the spring force if constraint conditions are pin and pin-roller constraint at center axis member. On the other hand, if the constraint condition is perfect constraint at longitudinal member, this means has no effect in suppression of the deformation at peripheral side of upper radial member, although the deformation at central side of it is almost suppressed. According to the thermal deformation analysis of the original model, deformation occurred towards normal direction of the reflector surface during thermal transition. Moreover, the midpoint of the LDR was confirmed to deform for about 5[mm] which explained the phenomena actually observed on the ETS-VIII.