The stochastic power fluctuation of ultra-high voltage（UHV）tie-line has become an important factor that restricts its transmission capacity.With the tie-line stochastic power fluctuation as the research object，an analysis is made of the significant effect of frequency deviation variation on stochastic power fluctuation by studying the total differential expression of tie-line transmission power.Then according to the system active power-frequency response characteristics，the mechanism of tie-line power fluctuation based on the load fluctuation is explored.At the same time，the dynamic process of tie-line stochastic power fluctuation from the perturbed response stage to the new steady-state stage is discussed.On this basis，with the introduction of the Monte Carlo test method，a probability distribution estimation method of the tie-line power fluctuation amplitude based on the two regionsload fluctuation characteristics is presented.This method uses probability statistics to estimate the probability distribution of interconnected regional load fluctuation amplitude，followed up by calculating the average deviation and standard deviation of power fluctuation amplitude.Calculation and analysis results of the probability distribution of North-Central China UHV tie-line stochastic power fluctuation amplitude have verified the validity of the proposed calculating method.