The invention discloses a method for preparing a sub-pixel structured planar InGaAs infrared detector chip, which comprises the following steps of: 1, extension material cleaning; 2, silicon nitride diffusion mask sedimentation; 3, first photoetching ; 4, sub-pixel diffuse window opening; 5, photoresist stripping; 6, closed pipe diffusion; 7, open-pipe piece taking; 8, second photoetching; 9, P electrode growing; 10, photoresist stripping; 11, silicon dioxide antireflection film sedimentation; 12, P electrode annealing; 13, third photoetching; 14, P electrode hole forming; 15, photoresist stripping; 16, fourth photoetching; 17, P electrode thickening; 18, photoresist stripping; 19, back side polishing; 20, N electrode growing; and 21, scribing. According to the invention, under the condition that the quantum efficiency of the prepared detector is not reduced, the photosensor can respond uniformly, the service life of the minority carrier is increased, and the dark current of the device is reduced; and due to the structure, as for a linear detector, the blind pixel rate can be effectively reduced, and the extension and cross talk of the photosensor are suppressed.