Rice is extremely sensitive to high temperature, especially at the flowering stage. Identifying new germplasm and breeding rice varieties with resistance to heat stress are therefore essential. After multi-year evaluations, a heat-resistant rice landrace D43 have been identified in our previous research. In this study, the relationship between heat resistance and flowering opening time (FOT) was analyzed both in the field and in phytotron. The results indicated that high temperature can accelerate flowering opening of different rice varieties; D43 showed early morning flowering (EMF) habit in different conditions, and FOT was mainly concentrated in a period of 8:30～10:00 AM under high temperature. The invariable high temperature resulted in a significant reduction of spikelet sterility during flowering opening. However, the EMF habit was conductive to avoid high temperature. Therefore, the spikelet fertility of D43 was relative high in the field and variable temperature treatment in phytotron. In addition, morphological traits including the anther adhesion rate, the total number and germinated number of pollens on the stigma were significantly correlated with the spikelet fertility, and therefore they can be used to evaluate rice heat resistant at anthesis.