Provided herein are improved processes and methods for preparing osimertinib or a salt thereof, in particular osimertinib mesylate. The improved process removes the necessity of isolating the unstable aniline intermediate of formula (III) and enables the direct coupling to form the amide product of formula (II):




The present invention is suitable for a large-scale production, avoiding the isolation of unstable intermediate, thereby providing osimertinib or a mesylate salt thereof in both high yields and high purity.