Abstract: Immunosuppressive drugs are mainly used to fight against immune rejection after organ transplantation, and have great potential to reduce the incidence rate and mortality of patients. However, some drugs have characteristics such as narrow therapeutic range, significant individual differences in pharmacokinetics, and interactions between drugs. The blood drug concentration is closely related to efficacy and toxicity. Therefore, detecting the concentration of immunosuppressants in the blood can fully understand the drug exposure in the patient's body, optimizing the dosage can maintain a stable blood drug concentration, which are of great significance in ensuring the effectiveness and safety of medication. Mass spectrometry, as an analytical tool, has the characteristics of high sensitivity, high specificity, high selectivity, high speed, and low sample-consumption. This article reviewed the applications of liquid chromatography tandem mass spectrometry and in situ mass spectrometry in the detection of immunosuppressants in blood. Liquid chromatography tandem mass spectrometry has the characteristics of high sensitivity and good repeatability. In situ mass spectrometry can directly detect blood samples without the need for sample pretreatment. Finally, the prospect of mass spectrometry technology in the detection of immunosuppressants in blood was discussed.