In order to explore the mechanical response and control mechanism of passive pile underpinning, a combined method of field monitoring and model test was adopted to carry out relevant researches based on the pile underpinning project of Guiyang Metro Line 3. The vertical displacement of piers, horizontal displacement of retaining piles as well as vault settlement and convergence of the tunnel section at the pile underpinning location were monitored and analyzed on the construction site. The whole construction process including foundation pit excavation, pile cutting and tunnel excavation was simulated via model test, and the axial force of underpinning piles together with the axial force and bending moment of initial tunnel support was measured.The results show that the deformation of piers presents a trend of "uplift-settlement-stabilization" with a maximum settlement of 2.61 mm; the horizontal displacement of retaining piles is also characterized by inward deformation followed by outward deformation; no obvious difference is found between the tunnel monitoring data and those of the ordinary tunnel section, which indicates that the pile underpinning effect is satisfactory. It is verified by the model test that the axial force of underpinning piles increases sharply after the existing piles are cut off. The final axial force of underpinning piles under the working condition of cutting two piles (e.g., -8.3 kN for pile Z-4) is larger than that under the condition of cutting one pile (-6.7 kN for pile Z-4), and the vault of initial tunnel support is deemed as the peak zone of axial force (maximum -48.7 kN) and bending moment (maximum 0.49 kN·m).It is concluded that the passive underpinning scheme is safe and reliable. The influence of pile cutting quantity on the mechanical behavior of underpinning piles and the optimization direction of tunnel vault support are clarified, which can provide references for the design and construction of similar pile underpinning projects in shallow buried limestone strata