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Recent breakthroughs in two-dimensional magnetism have attracted interest in exploring novel van-der Waals magnetic materials. While they exhibit exciting magnetic phenomenon in bulk form, the persistence of long-range magnetic ordering in atomically thin layers has become a promising direction to study magnetism in 2D limit. Here, we report the tunable magnetic structure in a new class of AFM van-der Waals Ni_1-xMn_xPS₃ system. Our study reveal a systematic evolution of spin-flop transitions by varying Ni:Mn content in Ni_1-xMn_xPS₃ single crystals, implying the re-orientation of magnetic moments of the 3d element. Such effective tunning provides a playground to engineer the 2D magnetism, offering promising candidates for new phenomena arising from 2D magnetism and future device applications.