Quad-rotor unmanned aerial vehicle (UAV) is a typical multiple-input-multiple-output underactuated system with couplings and nonlinearity. Usually, the flying environment is very complex, so that it is impossible for the UAV to avoid effects derived from disturbances and uncertainties. In order to improve the reliability of flight control, we established the dynamic model of quad-rotor UAV by Newton-Euler equation in unbalanced load conditions. Considering external disturbances in the attitude, a second-order sliding mode controller was designed with PID sliding mode surface and Extended State Observer (ESO). The simulation experiments have got good control performance, illustrating the effectiveness of our controller. Meanwhile, the controller was implemented in a quadrotor UAV, which carried a pan-tilt camera for aerial photography. The actual flight experiments proved that this paper dealt with the high stabilization flight control problem for the quad-rotor UAV, which laid a good foundation for autonomous flight of the UAV.