Abstract: The physical interaction in subcellular scale targets and DNA damage effect on a voxel model of lung epithelial cell from healthy human combined with DNA model self-constructed in Geant4 were performed for tritium radiation. The influence of single β particle on environment in cell and DNA at different remained states of tritium was analyzed. The results show that the source distributions of tritium varying from cellular scale to subcellular scale play an important role resulting in some regular variation on irradiation effect of different targets. When the nucleus is considered as source, the energy ratio difference of nucleus and ecytoplasm for two source-target combinations between N->N and N->C is the most notable, at the same time, the S_N->N as a character of microdosimetry is up to maximum. In addition, for single β particle of tritium, the size of cluster damage in DNA is less than 3, and the major type of cluster damage is simple single strand break (SSSB). Although the proportion of cluster damage (<5%) of cluster size more than 3 is very low, it is still possible to produce cluster of size more than 6. The yield of double-strand break (DSB) is 5% of that of single strand break (SSB), the yield of complex double-strand break (CDSB) is about 20% of that of DSB, and the yield of CDSB is maximum when the source is nucleus.