Abstract: Studies on the relative biological effectiveness (RBE) of tritium as nuclear fusion fuel under low-dose irradiation from the perspective of radiation protection were summarized. Two tritium irradiation methods, namely, exponentially decreasing dose rate and constant dose rate, were selected to observe and analyze the following biological indicators: dominant lethal mutation rate in oocytes and spermatocytes, dominant skeletal mutation rate, survival rate of primary oocytes and spermatogonia, and micronucleus cell rate in peripheral blood lymphocytes and polychromatic erythrocytes of fetal liver. The RBE under these two tritium irradiation methods was calculated, and the factors influencing the RBE were analyzed. Results showed that at cumulative doses of 0.2, 0.3, 0.4, 0.5, and 0.6 Gy/10 d, the RBE under the tritium irradiation methods of exponentially decreasing dose rate and constant dose rate was 2.9–4.2. For radiation protection, the RBE of low-linear energy transfer (LET) radiation on the biome must be set to 3.0–3.5. In case of exposure to tritium β particles or other low-LET radiation or values close to the derived reference level, a high RBE may be required for an accurate estimation of the radiation risk.