This paper highlights the use of the OpenMP4.5 accelerator programming model to collaborate CPUs and Intel Many Integrated Cores (MIC) co-processors for large-scale LBM multiphase flow simulationson the Tianhe-2 supercomputer. To enhance the collaborative efficiency among intra-node CPUs and co-processors, we propose a flexible load balance model with heterogeneous domain decomposition for CPU-MIC task allocation, as well as asynchronous offloading to overlap operations of CPUs and multiple MICs. Tests for 3D multi-phase (liquid and gases) problem (about 100 Billion lattices) simulating drop impact with gravity effect using D3Q19 Lattice Boltzmann discretization and Shan-Chen BGK single relaxation time collision model are presented, achieving a weak parallel efficiency of above 80% in going from 128 to 2048 compute nodes.