A new type of polyoxomolybdate containing mixed-valence arsenic, [AgAsIII2(AsIIIAsVMo4O18(OH)2)3]11− (1), has been synthesized. Single-crystal X-ray analysis was carried out on (NH4)5(C2H9N2)4.5H1.5[AgAsIII2(AsIIIAsVMo4O18(OH)2)3]·14H2O (1a) and (NH4)11[AgAsIII2(AsIIIAsVMo4O18(OH)2)3]·16H2O (1b). The polyanion is composed of three unprecedented [AsIIIAsVMo4O18(OH)2]6− subunits linked together by two AsO3 trigonal-pyramidal structures from both sides via sharing six bridging oxygen atoms, and a silver cation is enclosed in the center of the polyanion and coordinated by three As atoms with the As–Ag–As bond angle of 120°, to lead to a fan-like structure with an approximative C3h symmetry. The polyanion was also characterized by IR, X-ray photoelectron spectroscopy and thermogravimetry-differential scanning calorimetry. The stability of [AgAsIII2(AsIIIAsVMo4O18(OH)2)3]11− in an aqueous solution was investigated by electronic absorbance spectroscopy and electrospray ionization mass spectrometry, and the possible disaggregation mechanism of the polyanion is proposed.