In order to provide theoretical basis for the homogeneity request of heavy—ion irradiation field,the most important design parameter of the heavy —ion radiotherapy facility planned in IMP (Institute of Modern Physics) ,the influence of the homogeneity of heavy —ion irradiation field on the survival fraction of cells was investigated theoretically. A formula for survival fraction of cells irradiated by the un —uniform heavy —ion irradiation field was deduced to estimate the influence of the homogeneity of heavy —ion irradiation field on the survival fraction of cells. The results show that the survival fraction of cells irradiated by the un—uniform irradiation field is larger than that of cells irradiated by the uniform irradiation field, and the survival fraction of cells increases as the homogeneity of heavy —ion irradiation field decrease. Practically, the heavy—ion irradiation field can be treated as uniform irradiation field when its homogeneity is better than 95%. According to these results, design request for the homogeneity of heavy —ion irradiation field should be better than 95%. The present results also show that the agreement of homogeneity of heavy —ion irradiation field must be checked while comparing the survival fraction curves obtained by different laboratory.