The aim of the present work is the comparative analysis of data of neutron diffraction investigation of DMAAS ((CH3)2NH2Al(SO4)26H2O) single crystals in paraelectric and ferroelectric phases at 293 and 135 K, neutron spectroscopy investigation of polycrystalline samples of DMAAS in a wide temperature range above and below the ferroelectric phase transition point and theoretical calculations with the purpose of ascertainment of the structural phase transitions mechanism in DMAAS crystals.