The invention discloses a geosteering method for horizontal well while drilling natural gamma. The method comprises the following steps that: I, when a level driller drills, a gamma detector detects natural gamma rays in the drilled stratum in real time, and well recording equipment transmits the measured well incline angle 1 and well depth data 1 to a data acquiring, analyzing and processing system in real time; II, the data acquiring, analyzing and processing system makes a while drilling gamma graph and correspondingly judges the drilling-in and drilling-out trend of a drill bit: 1, makinga symmetry and balance line and 2, judging whether the gamma ray changing graphs on both side areas of the symmetry and balance line have symmetrical sections or not to determine that the drill bit drills out from one stratum and then drills into the same stratum is illuminated or that the drill bit drills out from one stratum and then drills into the adjacent stratum is illuminated correspondingly; and the well depth data of the drill bit position are combined to synthetically judge synchronously, and the drill bit drilling-in direction can be adjusted correspondingly. The method can correctly judge the drilling-in and drilling-out trend of the drill bit, with simple steps, and can effectively solve the problem that the drill bit can reliably pass through the subject stratum.