Abstract: 3-Aminopropylaminoethylthiophosphoric acid(Ⅰ) was shown to be able to diminish the side effect of radiotherapy of cancer, because it is selectively absorbed by healthy tissues but not by tumor. In this paper, an improved method of synthesis of some aminoalkylthiophosphoric acids is described.For example, dimethylsulfoxide(DMSO) can be used in place of dimethylformamide (DMF) to catalyse the reaction between N-(2-bromoethyl)-trimethylene-1,3-diamine dihydrobromide (Ⅲ) and trisodiumphosphorothioate to form compound(Ⅰ).The reaction rate with DMSO is faster than that of DMF. In other instances, the reacion between bromoalkylamine hydrobromide and trisodium phosphorothioate can proceed at a smooth rate without the catalyst.The synthetic compounds were tested for radio-protective activity in mice. Preliminary results showed that compounds (Ⅳ), (Ⅶ) and (Ⅸ) provided good protection against ionizing radiation in mice.