Ordered nanocomposite with one-dimensional periodic dispersion of inorganic nanoparticles in photopolymer has been extensively investigated in last decades for its potential applications in holographic data storage, nonlinear optics and holographic diffractive elements for slow-neutron, etc. Large refractive index modulation (Δn) is a crucial requirement for these applications. In this work, we focus on the study of Δn of polymer-nanoparticle composites based volume gratings. Mercaptoethanol capped zinc sulfide nanoparticles (ME-capped ZnS NPs) with ~5 nm diameter are one-pot synthesized from thiourea, zinc acetate and mercaptoethanol. Holographic mixtures are composited with the synthesized ME-capped ZnS NPs (36 wt% content), N,N-dimethylacrylamide, hyperbranched monomer and a novel photoinitibitor consisted of 3,3′-carbonylbis(7-diethylaminocoumarin) (KCD) and N-phenylglycine (NPG). The influences of KCD content and recording light intensity on the value of Δn have been studied. Under a 66.0 mW/cm2 recording light intensity, when the KCD content increases from 0.1 wt% to 0.6 wt%, the Δn of the prepared volume gratings increases from 10.4×10-3 to 17.8 ×10-3, increased by 71%. With a fixed 0.6 wt% KCD content, when the recording light intensity decreases from 66.0 mW/cm2 to 5.5 mW/cm2, the Δn increases from 17.8×10-3 to 26.6×10-3, increased by 49%. We conclude that both the increased KCD content and decreased recording light intensity would lead to a delayed formation of polymer network which enhances the diffusion of ME-capped ZnS NPs during the fabrication of the volume gratings. It is worth to mention that the Δn (26.6×10-3) achieved with our photopolymer-ZnS nanocomposite is higher than the previously reported highest Δn (24.0×10-3) in the field of polymer-inorganic nanocomposite based volume gratings.