Optimal investment time of lumpy capacity espansion investment under bistochastic uncertainties of price and capacity utilization rate was investigated in this paper. Applying theory of real options, it developed an investment decision-making model aimed to maximize investment options based on expected first hitting time. A simulation optimization algorithm designed by Riskoptimizer software package was used to solve the model. Numerical examples were carried out in the end which indicated that the biuncertainties＇ influences on optimal capacity expansion decisions