Different thick Ni layers are deposited on the GaN-based LED films grown on Si(111) substrates, then LED films are annealed at 400℃—750 ℃ in the atmosphere of N2 ∶O2=4 ∶1. The Pt / p-GaN contact layer is prepared after removing the Ni-capping layer. It is found that annealing temperature and thickness of Ni-capping layer each have an important influence on the p-type contact of GaN-based LED film. The Ni film can significantly reduce the activation temperature of Mg acceptor of the p-type GaN. The characteristic of p-type contact of Ni-capping sample becomes better first then turns worse with annealing temperature and it become better then turns worse and then better with Ni-capping thickness. After optimization, the specific contact resistivity of Pt/p-GaN in the case of no second annealing can reach 6.1×10-5 Ω·cm2, when Ni-capping layer thickness is 1.5 nm and its annealing temperatune is 450 ℃.