YBSW015 is a bispecific antibody that targets severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). A pair of high-affinity parental monoclonal antibodies (B38 and H4) were discovered from the sera of convalescent patients, and YBSW015 was developed based on these two parental antibodies. To support pharmacokinetic research, a simple, specific and reproducible liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed and validated for the quantifying YBSW015 in human serum. Two signature peptides (SPs) were selected as surrogate analytes for quantification and qualitative detection, respectively. Two stable isotope-labeled synthetic peptides with the same sequence of two SPs were set as internal standards (ISs). Total antibody was precipitated with 1.0 % (w/v) trichloroacetic acid (TCA) in isopropanol (IPA) following by trypsin enzymatic and further purified by solid phase extraction (SPE). The performance of this bioanalytical assay was well evaluated with respect to linearity, accuracy and precision, selectivity, specificity, matrix effect, recovery, and stability. YBSW015 concentrations in human serum were determined over the concentration range of 2.00-1000 μg/mL, and the quantification was not interfered by the matrix components and pre-existing antibodies. This precise and sensitive method was successfully employed in a phase I clinical study.