The invention belongs to the technical field of isocyanate and especially relates to a method for reducing content of chlorinated derivative in isocyanate and increasing the quality of isocyanate. The method disclosed according to the invention reduces the content of chlorinated derivative in isocyanate in the manner of enabling cyanate or thiocyanate to react with crude or fine product of isocyanate and then simply performing post-processing. The method can be used for removing more than 90% of chlorinated derivative from isocyanate and realizing the impurity content of the chlorinated derivative below 0.1%, is simple in operation and is easy for industrial application. Besides, the isocyanate treated according to the method is capable of improving the anti-ageing property of a downstream macromolecule product thereof due to the reduced content of the chlorinated derivative.