This paper proposes a fixed-time sliding mode attitude controller for quadrotor UAVs with prescribed performance constraints. The controller guarantees that tracking errors evolve within predefined, time-varying bounds and converge to zero in fixed time, independent of initial conditions. A performance function is used to impose transient and steady-state constraints, and an error transformation maps the constrained dynamics into an unconstrained space. A nonsingular sliding surface and control law are designed to ensure global fixed-time stability. Lyapunov analysis confirms the theoretical properties. Simulation results verify that the proposed method achieves accurate tracking while strictly satisfying performance bounds, demonstrating its effectiveness for constrained attitude control.