The invention relates to a mobile automatic switching and fast sampling flue gas denitrification testing system. The testing system comprises a testing trolley, equipment loaded on the testing trolley, and a control device. The equipment comprises multiple flue gas sampling steel pipes arranged through a grid method, an on-site flue gas sampling and switching device communicated with the flue gas sampling steel pipes, a middle flue gas sampling and switching device communicated with the on-site flue gas sampling and switching device, a flue gas draw-off pump communicated with the middle flue gas sampling and switching device, and a flue gas analyzer communicated with the draw-off pump. According to the improvement of the testing system, automatic switching and fast sampling testing of the grid method can be carried out on flue gas of 220 points under one testing working condition can be achieved within 25 min, and influence on testing results from unit loads and combustion fluctuations is effectively reduced; in addition, the process facility is simplified, and the phenomenon that traditionally, a long flue gas pipeline is blocked due to water vapor condensation is effectively eliminated.