Abstract: mRNA therapy, which involves the use of mRNAs as drugs for disease treatment, is a new kind of gene therapy. It can either treat diseases caused by gene deficiency or repair tissue through the expression of functional proteins, or be applied to immunotherapy through the expression of antigens, antibodies, or receptors, and is thus, of great value for various clinical applications. In tumor immunotherapy, mRNA that encodes tumor-related antigens, or specific antigens, antibodies, or receptors enters the cytoplasm and is translated into proteins, which then induce specific immune responses, thereby enabling disease prevention and treatment. With the development of immunotherapy and mRNA technologies, mRNA therapy for malignant tumors and infectious diseases has entered the stage of clinical applications. This review briefly introduces the synthesis, purification, and modification of mRNA, with emphasis on mRNA-based tumor immunotherapy, clinical trial results, and key opportunities and challenges in the development of new drugs.