Optical resonators with high quality ($Q$) factors are paramount for the enhancement of light–matter interactions in engineered photonic structures, but their performance always suffers from the scattering loss caused by fabrication imperfections. Merging bound states in the continuum (BICs) provide us with a nontrivial physical mechanism to overcome this challenge, as they can significantly improve the $Q$ factors of quasi-BICs. However, most of the reported merging BICs are found at $\mathrm{\Gamma }$ point (the center of the Brillouin zone), which intensively limits many potential applications based on angular selectivity. To date, studies on manipulating merging BICs at off-$\mathrm{\Gamma }$ point are always accompanied by the breaking of structural symmetry that inevitably increases process difficulty and structural defects to a certain extent. Here, we propose a scheme to construct merging BICs at almost an arbitrary point in momentum space without breaking symmetry. Enabled by the topological features of BICs, we merge four accidental BICs with one symmetry-protected BIC at the $\mathrm{\Gamma }$ point and merge two accidental BICs with opposite topological charges at the off-$\mathrm{\Gamma }$ point only by changing the periodic constant of a photonic crystal slab. Furthermore, the position of off-$\mathrm{\Gamma }$ merging BICs can be flexibly tuned by the periodic constant and height of the structure simultaneously. Interestingly, it is observed that the movement of BICs occurs in a quasi-flatband with ultra-narrow bandwidth. Therefore, merging BICs in a tiny band provide a mechanism to realize more robust ultrahigh-$Q$ resonances that further improve the optical performance, which is limited by wide-angle illuminations. Finally, as an example of application, effective angle-insensitive second-harmonic generation assisted by different quasi-BICs is numerically demonstrated. Our findings demonstrate momentum-steerable merging BICs in a quasi-flatband, which may expand the application of BICs to the enhancement of frequency-sensitive light–matter interaction with angular selectivity.