This study simulates particle-laden turbulent flow in a horizontal open channel using direct numerical simulation and the Lagrangian point-particle tracking method. The influence of inter-particle collisions, under the action of different gravitational effects, mean particle volume fraction and particle inertia, on two-phase interactions is investigated using a hard-sphere model and a neighbor-cell searching algorithm. The simulation results show that at low volume fractions, regardless of whether gravity is considered, collisions have little effect on particle-induced turbulence modulation. At high volume fractions, collisions of inertial particles mainly occur in low-vorticity regions near the wall. Compared to the two-way coupling case, these collisions promote the generation of extra velocity fluctuation and enhance turbulent fluctuations. Under the influence of gravity, however, particle collisions mainly take place in high-vorticity regions near the wall. These collisions suppress the wall rebound of heavy particles and the ejection of near-wall fluid, disrupt low-speed streaks and hairpin vortices, and lead to a further reduction in turbulent fluctuations. As particle inertia increases, the attenuating effect of collisions among heavy particles on turbulent fluctuating velocities becomes more pronounced.