Abstract: Near-Coulomb-barrier (near-barrier) fusion of heavy-ions is a complicated reaction process which is related to the quantum tunneling and coupled-channels effect as well as the synthesization of super-heavy elements. It has been a hot topic since the discovery of sub-barrier fusion enhancement phenomenon in 1978. The proposed concept of fusion barrier distribution has stimulated many high-precision experiments and pushed the understanding of the coupled channels involved in the near-barrier heavy-ion fusion process. The relevant effects of low-lying inelastic excitations have been highlighted based on this idea. Among the coupled channels involved in the heavy-ion fusion process, coupling to nucleon transfers is a complicated problem. Up to now, the experimental conclusions for the transfer effects are still controversial and the theories still can’t deal with this coupling self-consistently. For studying this mechanism experimentally, the neutron-rich ¹⁸O-induced fusion with medium-mass target is an ideal choice, since this type of system usually has a big positive Q-value two-neutron (2n) transfer channel and simultaneously has no positive Q-value proton stripping channels, wherein both the two factors may contribute to the sub-barrier fusion enhancement. For this, the ^16,18O+⁵²Cr systems were selected, wherein ¹⁸O+⁵²Cr was measured for studying the effect of positive Q-value 2n stripping channel while ¹⁶O+⁵²Cr for a reference. The experiment has been performed by using the electrostatic deflector at near-barrier energy region at the R60º nuclear reaction terminal of the Beijing HI-13 Tandem Accelerator at the China Institute of Atomic Energy. The fusion evaporation residues at forward small angles were separated from the scattered beam-like particles, and then identified based on the time-of-flight and energy signals of the particles, that is the TOF-E method. The experimental data were analyzed by using the coupled-channels code CCFULL. The measured fusion excitation function of ¹⁶O+⁵²Cr can be well reproduced by the CCFULL calculations. While, for ¹⁸O+⁵²Cr, the calculated results including both the 2₁⁺ vibrational state of ¹⁸O and 2₁⁺ vibrational state of ⁵²Cr somewhat underestimate the sub-barrier experimental result. Combing with the available experimental results of ¹⁸O+¹¹⁸Sn, ⁷⁴Ge, a systematic conclusion of no remarkable sub-barrier fusion enhancement for the systems with positive Q-value 2n-stripping channel can be tentatively obtained. This finding offers a challenge for the prevailing argument that the sub-barrier fusion of heavy-ions is greatly enhanced due to strong coupling to the positive Q-value neutron transfer channels. The present result offers a critical restriction for the current theoretical models, and whilst the experimental studies for more systems are needed to extend this systematics.