The remote sensing system fitted on UAV (unmanned aerial vehicle) can obtain clear images and high-resolution aerial photographs. It has advantages of strong real-time, flexibility and convenience, little influenced by the external environment, low cost, low-flying under clouds and ability to work full-time. It can go deep into the places safely and reliably which staff can hardly approach. The system can be timely in response to emergency monitoring and assessment by way of obtaining first-hand information as quickly as possible, producing a unique emergency response capacity to provide a scientific basis for overall decision-making. It is highly suitable for handling contingency working model and social unexpected events of disaster relief at present, especially suitable for the use of on-site contingency response team. It can greatly enhance the capability of disaster emergency working team on-site in data collection and transmission. The great advantages of UAV remote sensing system have played an irreplaceable role in monitoring and evaluating natural disaster. Taking Wenchuan Earthquake as an example, UAV high-resolution remote sensing images have been exploited to estimate the situation of disaster-hit areas and monitor secondary geological disasters rapidly, systematically and continuously such as landslides and barrier lakes. Subsequently, a rapid quantitative assessment on the distribution and size of landslides and barrier lakes is carried out. At the same time, the loss assessment such as buildings, roads, and bridges are also described. The monitoring and evaluation results along with related reports are presented to the relevant government departments and rescue teams, which can provide a detailed and reliable scientific basis for disaster relief and reconstruction.