Disclosed are rice environmental conditional-lethal mutant gene osesl1, an encoding protein and use thereof The gene osesl1 has a nucleotide sequence shown as SEQ ID NO: 1 in the Sequence Listing. The encoding protein thereof has an amino acid sequence shown as SEQ ID NO: 2. After heading of osesl1 mutant rice, seed embryo lethal phenotype appears at 12 days after pollination, exhibiting darkening at the junction between embryo and endosperm. When an average temperature is below 22° C., a seed embryo is normal; when the average temperature is above 28° C., the seed embryo is lethal; when the temperature is between 22° C. and 28° C., the seed embryo is lethal under long daylight conditions (>13 h) and normal under short daylight conditions (