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In recent years,double spike has been widely applied in the analysis of molybdenum isotopic ratios.The key factor controlling the precision of the double spike are the selection of double spike composition(λ)and the proportions of double spike to sample in mix(p).Above parameters can be obtained by the fitting of measured data or the computer numerical simulation.In this paper,we used a new method which was based on Monte Carlo and Nest iterative algorithm,at the same time,combined with computer numerical simulation to get the optimal ratio of molybdenum isotope double spike.Molybdenum has seven stable isotopes in nature,which are 92Mo(14.836%),94Mo(9.247%),95Mo(15.920%),96Mo(16.676%),97Mo(9.555%),98Mo(24.133%),100Mo(9.633%)respectively,so double spike choices may have 21 pairs.But the application of 97Mo-100Mo double spike is more common.The optimization of 97Mo-100Mo double spike includes two aspects: ① is to optimize the ratio between 97Mo and100Mo single spike; ② is of optimizing proportion of 97Mo to 100Mo in mix.Before the optimization of double spike,we required to design an error model,and combined with the Monte Carlo algorithm to product a range of measured values,and these measured values were corrected by Nest iterative algorithm.Finally,we can obtain the optimizing double spike by the size of the error propagation value.The best proportion between two single spikes(97Mo:100Mo)ranges from 0.25-1.5.For the convenience of calculation,we choose(97Mo:100Mo)spike=1 as the best proportion.we also can obtain the minimum value of the error of fractionation factor(error in α)when the proportion of double spike in mix is equal to 0.5.The best proportion between double spike and sample is about 2.7(100Mospike :98Mosample ≈2.7)by calculating accordingly.