The hedge fund manager is often faced with the uncertainty of asset prices, where uncertainty includes both the classical probabilistic uncertainty and the Knightian uncertainty. We know that asset prices can be addressed by stochastic differential equations disturbed by a Brownian motion in the classical sense. However, due to the complexity of financial markets, it might be more reasonable that the interference sources of asset prices are characterized through Peng's G-Brownian motion. This paper investigates the optimal strategy of the fund manager's portfolio as the volatility of the asset price has Knightian uncertainty. First, we establish a dynamic model in which a fund manager invests in a riskless asset and a risky asset under the framework of G-Brownian motion. On the other hand, for the hedge fund with the contract of high water marks, the fund manager wants to maximize the expected net present value of the cumulative incentive fees. Then we deduce the corresponding G-Hamilton-Jacobi-Bellman equation of the value function with specific boundary conditions through the stochastic calculus and the stochastic dynamic programming method  under nonlinear expectations, and the corresponding optimal portfolio strategy of the fund manager is obtained. Finally, we give the static economic analyses for our results.