Provided is a composition of a ready-to-use injectable solution comprising pemetrexed or pharmaceutically acceptable salts thereof, containing pemetrexed; anhydrous sodium sulfite; and N-acetyl-L-cystein. With respect to use as an injectable preparation. The ready-to-use injectable solution composition is suitable for effective administration of pemetrexed since a melting process of main ingredients is not necessary before administration, and the composition can be used by being diluted immediately in a perfusate. In addition, the properties of the composition do not change even in long-term storage, and pharmaceutical stability is significantly improved by inhibiting related substances below a reference, whereby the composition can be stored in a liquid preparation state.