Aiming at the problem of intercepting maneuvering targets with impact angle constraints, an adaptive guidance law is proposed based on immersion and invariance (I&I) nonlinear control method. The mathematical model of the interception problem is established by considering the synthetic action of target maneuver as system disturbance. The process of guidance law design is divided into two steps. Firstly, an I&I disturbance observer to estimate the system disturbance is designed. Secondly, an I&I guidance law by taking the observer tracking error into consideration is designed. Then the closed-loop system stability of the guidance system is proved based on input-to-state stability theory. The guidance command is smooth for no switch function is involved. The designed guidance law guarantees the fast convergence of the line of sight angular rate, the robustness of resistance against target maneuver and the requirement of impact angle constraints. The simulation results show the effectiveness of the presented guidance law.