It is difficult to find a rapid and accurate method for the analysis of thorimn in monazite. Through a thorough investigation on the properties of elements in monazite and the characteristics of polarometric titration, it is able to combine steps of decomposition, separation and polarometric titration more properly. A dried sample is fused with potassium hydrogen fiuoride; by selecting a mixture of aluminum chloizde, acetic acid and sodium acetate as auppouing electrolyte, thorium can be rapidly and accurately determined by polazometric titration using ammonium molybdate as reagent. The optimum conditions are as follows: appliod voltage 0.75V (S. C. E.), composition of supporting electrolyte; 1.5 N AlCl₃, 7% CH₃COOH and CH₃COONa (pH=1.7). The precision of the determination lies within±2%.It takes three hours to carry through the determination.