The time-dependent density functional theory (TD-DFT) B3LYP method is used to investigate the UV-vis absorption spectra of six cyclometalated platinum complexes with long-chain β-diketonate ancillary ligands. The results indicate that the electronic transition from the ground state to the first excited state is ligand-to-ligand charge transfer (LLCT) and metal-to-ligand charge transfer (MLCT) transition. The maximum absorption wavelength of the six compounds is around in the range of 402~405 nm due to the near ultraviolet region. The finite field (FF) method is used to investigate the third-order nonlinear optical properties of the six complexes. The results show that these complexes display good third-order nonlinear optical coefficients (in 105 order of magnitude). From one to fourteen carbon atoms range, increasing β-diketonate carbon chain length and introducing methyl respectively in the ligands benzene or pyridine are both conducive to increase the third-order nonlinear optical properties.