Piezocatalysis is a promising technology to address environmental pollution by converting mechanical energy into chemical energy. Herein, MoSe2 nanosheets with different 1T phase percentages (ranging from 31–80%) were constructed by adjusting hydrothermal temperature. Moreover, the roles of phase engineering in the piezocatalysis were thoroughly investigated by degrading Rhodamine B and reducing Cr (VI) in ultrasonic vibration. The experimental results indicated that MoSe2 with the higher percentage 1T phase contributes to better conductivity, the higher polarization, more active centers and better piezocatalytic performance. In particular, the observed kobs constant and the ultrafast degradation rate k of MoSe2 prepared at 220 ℃ (MoSe2-220) with the highest percentage 1T phase (80%) are the fastest among all reported catalysts to date. The ultrahigh activity is attributed to the establishment of an internal electric field at the 1T and 2H phase boundaries, which drives the segregation of electron-hole pairs in MoSe2-220 nanosheets. Furthermore, ·O2− and ·OH as main reactive species were confirmed and a rational mechanism was finally put forward. This study offers a clear understanding of phase engineering in piezocatalysis and provides an efficiency strategy to construct highly efficient piezocatalysts.