The invention relates to a construction method of a deep foundation pit under complicated working conditions. The construction method comprises the following steps: (S1) earth is excavated to reach the elevation of a first underground layer; an envelop enclosure of the deep foundation pit is constructed; a first underground layer horizontal structure is constructed; the first layer horizontal structure is fixedly connected with the envelop enclosure; and a first soil outlet is formed in the first underground layer horizontal structure; (S2) two layers of earth are downwards excavated; horizontal structures of the layers are constructed at the bottoms of the layers, and are fixedly connected with envelop enclosure; and soil outlets corresponding to the layers are formed in the horizontal structures of the layers; (S3) a horizontal structure of a previous layer of the layers is constructed; and a soil outlet of the previous layer of the layers is formed in the horizontal structure of theprevious layer of the layers, and is corresponding to the first underground soil outlet; (S4) the steps (S2) and (S3) are repeated; soil is discharged through the soil outlets formed in the horizontal structures of all the layers; the sizes of the soil outlets of all the layers are gradually increased from top to bottom; the bottom designed elevation of the deep foundation pit is excavated to construct a baseplate; and (S5) the soil outlets of all the layers are closed in sequence from bottom to top.