This paper explores efficient and stable optical trapping using quasi-bound states in the continuum (quasi-BICs) in photonic crystal slabs. By breaking inversion symmetry by transforming square holes into isosceles trapezoidal holes, we create quasi-BICs with finite quality factors. Calculations show that optical forces at enhanced electric field locations effectively trap particles, with significant potential wells at these sites. Multipole expansion analysis indicates that particles at sharp corners suppress radiation and enhance trapping stability. Moreover, an increased number of trapped particles causes a redshift in resonance frequency and strengthens optical forces, especially at sharp corners. These insights are crucial for designing photonic crystal slabs for practical optical trapping applications, where positioning more particles at sharp corners improves quasi-BIC mode and trapping efficiency.