Abstract: In order to study if the settlement and displacement created by the first loading process of the reconstruction of large channel will have great effect to irrigation requirements, based on the deformation observation situ test of large trapezoidal channels arc bottom concrete seepage-proofing channel loading and unloading process, the author analyzed the deformation characteristics of the reconstruction large channel bed after the process of loading and unloading for the first time. The results showed that: in the process of loading and unloading for the first time of reconstruction large channel, would happen to a large elastic deformation and plastic deformation, the channel settlement and spring back amount in the vertical had certain characteristics with the external load changing; The subsidence displacement of the large reconstructed channel which created by the first loading process had larger relationship with the unsaturation soil thickness which below channel bottom; due to the channel in the process of loading and unloading for the first time had a larger settlement in vertical, to satisfy the irrigation conditions, should take some necessary measures in the channel design and construction.