A novel and facile strategy in developing a water stable magnetic metal-organic framework nanocomposite (Fe O @MOF) is herein reported, in which a Keggin polyoxometalate, phosphotungstic acid (HPW), was encapsulated within the MOF framework via one-pot synthesis method. The combination of HPW-embedded MOF and Fe O endowed the composite with high surface area, strong UV absorption, good hydrophilicity, and enhanced water stability. With these unique properties, the Fe O @MOF embedded HPW served as adsorbent as well as matrix for SALDI-MS (surface-assisted laser desorption ionization mass spectrometry) analysis of polar and non-polar compounds. The synergistic effect of Fe O and MOF showed an interference-free background at low mass region than the pristine MOF or Fe O counterparts. This simple approach can be used as new platform in developing magnetic MOF composites without the time consuming and labor-intensive preparation.