Human body’s subjective perception of environmental vibrations can be altered by introducing another vibrational stimulus—the vibration masking effect. Understanding the impacts of the masking effects on human vibration perception is theoretically relevant for optimizing environmental vibrations, regulating vibration perception, and enhancing human-factor ergonomics. A subjective evaluation experiment on human vibration perception was conducted, investigating the frequency-domain masking effect of vertical vibration on seated human subjects. A novel evaluation metric—the masking beneficial index(MBI)—and its calculation method were proposed, revealing the influence laws of key variables, such as the amplitude ratio between the masking vibration and the target vibration and their frequency difference, on the masking effect. Results reveal that under constant amplitude ratios, masking efficiency increases with the frequency difference when masking vibrations induce lower discomfort than target vibrations, thereby correlating with reduced discomfort perception of target vibrations. Under fixed frequency differences, an optimal amplitude ratio maximizing masking efficiency is observed, which increases with the target signal amplitude, whereas the efficiency at this ratio decreases with the increasing amplitude, demonstrating nonlinearity in human vibration perception.