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A new polyphosphate K 2 Sr 4 (PO 3 ) 10 is synthesized by a high-temperature solution method. This compound crystallizes in the triclinic space group of P 1̅, consisting of the 1D infinite [PO 3 ] ∞ chains and K and Sr ions between the chains. Compared with AM 2 (PO 3 ) 5 (A = K, Rb, Cs; M = Ba, Pb), K 2 Sr 4 (PO 3 ) 10 exhibits a more complex [PO 3 ] ∞ chain structure and more diverse metal cationic coordination environment. More importantly, K 2 Sr 4 (PO 3 ) 10 has both a deep-UV cutoff edge (<200 nm) and a significantly enlarged birefringence. First-principles calculations indicate that the birefringence of K 2 Sr 4 (PO 3 ) 10 is 0.017 at 1064 nm, about 2 times that of RbBa 2 (PO 3 ) 5 (0.008 at 1064 nm), which reaches a new height among the reported mixed alkali metal and alkaline earth metal phosphate. Theoretical calculations and structural analyses show that the enlarged birefringence of K 2 Sr 4 (PO 3 ) 10 mainly originates from the [PO 3 ] ∞ chains arranged in an inverted zigzag. This discovery introduces a new strategy for devising novel phosphate deep-UV optical crystals with a large birefringence.