A tandem mass spectrometer is provided in the present invention. The mass spectrometer includes an ion source, a quadrupole mass filter located at downstream side of the ion source, a linear ion trap disposed at downstream side of the mass filter and an ion detector placed on the side of the ion trap, all of which are placed in a vacuum environment. The instrument can obtain MS meeting the standard spectral library search criteria by the quadrupole mass filter cooperating with linear ion trap, realize any multi-stage MS under two modes of axial collision and resonance excitation, and predict and optimize the inflow amount and types of samples under the ion trap analysis mode by the quadrupole. A tandem MS analysis method is also provided, which can repeatedly provide precursor ion selection, ion acceleration, achieve high-energy collision dissociation, low product ion mass discrimination effect.