The invention discloses a foundation pit deslagging construction method. The method comprises following steps that firstly, a hydraulic winding drum (4) is arranged on a large arm (5), a steel wire rope (8) passes through a fixed pulley (7), and two ends are connected with a mechanical grab bucket (13) and the hydraulic winding drum (4); secondly, an excavator is moved to the edge of the top of afoundation pit (14), a signal enabling the mechanical grab bucket (13) to be opened is sent out through a remote control emitter (2), and the mechanical grab bucket (13) is controlled to be opened; thirdly, a descending switch of the hydraulic winding drum (4) is started, and the mechanical grab bucket (13) descends; fourthly, an ascending switch of the hydraulic winding drum (4) is started, and the mechanical grab bucket (13) is closed to grab soil sediments (15); fifthly, the large arm (5) and a small arm (6) are rotated to the position outside the foundation pit (14), and the mechanical grab bucket (13) is opened to finish unloading of the soil sediments (15); sixthly, the above steps are repeated to finish foundation pit deslagging. The method is safe, feasible, efficient, practical and suitable for use and popularization of foundation pit deslagging construction.