The invention relates to an additive for a lithium secondary battery electrolyte and a preparation method of the additive. The additive is Li[P(C<2>O<4>)<2>G<2>], wherein G is alkoxy, alkylthiol, an alkenyl-containing chain group, an alkenyl-containing annular group, a chain group containing non-terminal alkynyl, an annular group containing non-terminal alkynyl, a nitryl-containg chain group, a nitryl-containg annular group, phenyl, a phenyl derivative group or a heterocyclic group. By the additive for the lithium secondary battery electrolyte, the capacity and the cycle property of the battery can be integrally and effectively improved.