Abstract: This paper mainly focuses on analysis of ²³⁴Th in seawater by using MnO₂ co-precipitation and β counter. In lab study, several conditional experiments, such as the selection of filter membrane, pH, whether acidification before co-precipitation, carbonate concentration and the usage amount of MnO₂, were performed. If the mixed cellulose ester membrane was used for sample counting on gas flow β counter, it makes the blank count rate to be around (0.50±0.04) min^-1. Acidification can promote the precipitation of uranium because it can destory the complexation of uranium with CO₃^2-. At pH=10.0, the separation effect of ²³⁴Th from ²³⁸U solution is good. Moreover, carbonate can coordinate uranium and make uranium kept in solution. Besides, the efficiency is increased when more MnO₂is added. The best reagent amount is 1.2mg KMnO₄ and 3.0mg MnCl₂·4H₂O per liter of sample in this study. We have applied this method combined with the actual situation to analyze ²³⁴Th in seawater samples from East China Sea. The recovery of ²³⁴Th is around （36.0±5.2)%(n=3).