The invention provides a method for caculating displacement of a nearby existing subway tunnel due to foundation pit excavation. The method specifically comprises the following steps of establishing a calculation model first, performing unloading analysis on the pit bottom, then performing unloading analysis on the side walls of a foundation pit, and finally, obtaining the horizontal total displacement Sx and the vertical total displacement Sz, which are caused by unloading stress generated by the pit bottom and surrounding side walls of the foundation pit, of a certain point (x1, y1, z1) on the outer side of the tunnel by utilizing a superposition principle; in order to accurately calculate the outside displacement of the tunnel under the action of loads of the pit bottom and surrounding side walls of the foundation pit, the half-side tunnel towards the foundation pit is uniformly divided into a plurality of points, displacement values of the plurality of points are respectively calculated, and finally, an average displacement value is used as the displacement value of the tunnel.Therefore, the construction of a specific project can be simulated by using the method before construction so as to calculate the displacement of the subway tunnel in specified working conditions, and if the displacement of the tunnel exceeds a corresponding allowable value, related construction parameters can be adjusted for trial computation till reaching safety standards.