The invention discloses an observational method used for a trench slope gravitational erosion field test. The observational method is characterized in that a DV sight line is perpendicular to structured light rays on an abrupt slope through a geography instrument, and according to video screenshots in the test process, the gravity erosion amount and the waterpower erosion amount, namely the total erosion amount, the gravity erosion amount and the waterpower erosion amount of individual rainfall, in an individual rainfall event, the gravity erosion amount occurring after rainfall is completed, and slope direction distribution and gradient distribution of the abrupt slope at any time can be respectively calculated. The observational method is further characterized in that the definition of laser rays projected on the abrupt slope is strengthened by adjusting indoor light rays, building a rainproof observation room and installing an anti-fog filter, the DV sight line is perpendicular to a laser plane emitted by a geography instrument main machine by adjusting the sight line angles of a DV, and testing and adjustment are carried out according to a sight line calibrator. According to the observational method, the water power erosion amount can be separated from the gravity erosion amount, and the gravity erosion amount in the rainfall process and the gravity erosion amount after rainfall can be quantitatively observed and calculated.