The invention provides a cathode additive, a preparation method thereof, a cathode sheet and a lithium battery. The cathode additive includes a composite material formed by an inorganic lithium salt and a conductive agent. The inorganic lithium salt includes a substance with the general formula being Li<x>MNO<y>, wherein 2<=x<=8, 0<a<=1, 0<=b<1, a+b>=1, 2<=y<=6, the M is one selected from Fe, Co, Mn, Ni, Zr, V, Nb and Mo, and the N is one selected from Al, Mg, Ti, Cr, Y, Sr, Si, W, Ga and Zn. The cathode additive can improve the first discharge capacity of the lithium battery, reduce thecharge transfer impedance of the lithium battery, and improve the energy density, cycle performance and rate performance of the lithium battery.