Summary: The core of this paper is to put forward a new method of investment decision-making and apply it to the practice of mine investment. Taking the net present value as the objective function and the prediction of price and cost as the center, the method combines the fuzzy interval grey prediction method with the geometric Brown motion stochastic process together. The transformation method of fuzzy number to interval grey number is given firstly. Then, the grey interval prediction model is established based on the idea of forecasting the upper and lower limits of interval sequence respectively. To obtain operating cost, key Brownian motion coefficients are obtained by path matrix, combination experiment, visual comparison and manual analysis. Finally, through a copper investment project, the proposed method based on fuzzy interval GM \((1, 1)\) prediction model and Brownian motion stochastic process theory is used to predict the metal price and operating costs successfully. As a result, risk assessment of metal mine is realized and feasible decision-making is formed.