We investigate the optical trapping of polystyrene microspheres in optical tweezers. The transverse capture gradient forces of polystyrene microspheres with different numerical aperture are theoretically and experimentally evaluated by the power spectral density roll-off method. It is found that the trapping force of the experimental measurement is much stronger than that of the theoretical results. The discordance is attributed to the slow light effect near the focus, which has been found in recent years [Science347, 857 (2015)10.1126/science.aaa3035; Opt. Express18, 10822 (2010)10.1364/OE.18.010822; Opt. Commun.332, 164 (2014)10.1016/j.optcom.2014.06.057]. The modified trapping force of the theoretical results by considering the slow light effect near the focus is well consistent with that of the experimental results.