Achieving low-threshold and highly stable nanolasers remains a major challenge for photonic integrated circuits and on-chip light sources. Bound states in the continuum (BICs), owing to their theoretically infinite quality factors (Q-factors), offer an effective strategy to boost light-matter interactions for low-threshold lasing. However, ideal BICs are decoupled from external radiation fields, requiring symmetry breaking to transform them into quasi-BICs for practical excitation. Nevertheless, the Q-factors of quasi-BICs are exquisitely sensitive to structural parameters, hindering performance preservation under real-world fabrication conditions. To overcome this limitation, we propose a merging BIC design based on a trapezoidal grating with an indium phosphide (InP) gain medium to realize low-threshold, highly robust nanolasers. The band structures and Q-factors were calculated using the finite element method (COMSOL Multiphysics), and the lasing dynamics were investigated via the finite-difference time-domain (FDTD) method. By breaking the vertical symmetry of the grating, multiple BICs are merging in momentum space, resulting in ultrahigh-Q resonances (Q > 10⁵) over a broad momentum range of Δk ≈ 0.13. At a central lasing wavelength of 838 nm, the lasing threshold of the trapezoidal grating merging BIC nanolaser is as low as 0.816 μJ/cm², representing an 18% reduction compared to its rectangular grating counterpart (1.000 μJ/cm²). Furthermore, tolerance analysis indicates that under typical fabrication imperfections—such as 10 nm-level dimensional deviations and sidewall roughness—the proposed structure maintains high-Q resonances with the lasing threshold strictly controlled below 1.320 μJ/cm², demonstrating superior fabrication tolerance and perturbation immunity. These numerical findings highlight the strong potential of trapezoidal grating merging BICs in designing low-threshold, highly stable on-chip nanolasers.