With the addition of Si to replace some P, Pd_40.5Ni_40.5Si_xP_19-x (x=0, 2.5, 5, 9.5, 14, and 19 in atomic number fraction) bulk glassy samples with the diameter of about 5 mm were successfully prepared by use of flux treatment and water quenching technology. With the increase of Si content, the glass forming ability of Pd_40.5Ni_40.5Si_xP_19-x increases first for low Si content and then decreases for high Si content (Si≥9.5at%). The Pd_40.5Ni_40.5Si₅P₁₄ glassy alloy possesses the largest supercooled liquid region ΔT of 119 K, the largest reduced glass transition temperature of 0.621, and the largest γ parameter of 0.460, indicating that this glassy alloy possesses very large glass forming ability and very high thermal stability.