The invention discloses a deep foundation pit construction method for controlling the foundation pit deformation under a diagonal brace supporting system. The deep foundation pit construction method for controlling the foundation pit deformation under the diagonal brace supporting system comprises the steps of excavating earthwork of a diagonal brace part of the diagonal brace supporting system of a deep foundation pit; constructing a first cushion layer on the diagonal brace part; constructing a first foundation slab on the diagonal brace part to seal the the diagonal brace part; excavating earthwork of a non-diagonal brace part of the diagonal brace supporting system of the deep foundation pit; constructing a second cushion layer on the non-diagonal brace part; constructing a second foundation slab on the non-diagonal brace part to seal the the non-diagonal brace part, and further sealing the whole deep foundation pit. According to the deep foundation pit construction method for controlling the foundation pit deformation under the diagonal brace supporting system provided by the invention, the earthwork of the diagonal brace part is excavated firstly, and then the cushion layer and the foundation slab of the diagonal brace part are quickly sealed, so that the function of temporarily propping the foundation pit internal force can be realized, the non-brace time of the foundation pit is reduced, the exposure duration of the foundation pit is reduced, and the foundation pit deformation is controlled maximally.