This paper studies a principal-agent problem in continuous time with ambiguity, an uncertainty except probabilistic uncertainty with the known probability distribution. The effects of an agent's moral hazard on the execution and lasting of a contract are studied. Firstly, the dynamic equations of the agent's continuation value as well as the principal's expected profit are established. Then, according to the theory of stochastic optimal control and Peng's sublinear expectation theory, the corresponding Hamilton-Jacobi-Bellman ( HJB) equation of the principal's value function, as well as the expression of the principal's optimal payment and the agent's optimal effort level, is constructed. Finally, numerical simulations are provided to explain the effects of Knightian uncertainty on the optimal dynamic contract and the two parties' optimal strategies.