Abstract Reformation of the power industry makes generation companies become independent investors in power market, this article studies investment decisions of generation companies. On the basis of basic investment principle that generation companies always are in pursuit of maximum profit from investment, it is analyzed how power price fluctuation property affects the timing and the probability of generation capacity project investment exercise through martingale and change of measure in this article. The studies show that generation companies generally delay investing under energy price uncertainty, and capacity price has much incentive to invest in capacity project. The conclusion indicates that uncertainty is the cause of investment risk, increase in uncertainty of energy price raises investment risk of generation companies, and reduces the probability of generation project investment exercise.