We report the superconductivity enhancement of ZrTe3 on compression up to 33 GPa. The superconducting transition occurs above 4.1 GPa and the superconducting temperature (T C) increases with pressure in further compression, reaching a maximum of 7.1 K at ~28 GPa. An anomalous change of superconducting temperature is seen in the compression above 21 GPa. No structural phase transition is observed in the whole compression up to 36 GPa, but a subtle change in structural parameter is seen between 17-19 GPa, which seems relevant to the anomalous increase in the superconducting temperature. First-principle calculations reveal that the density of states at the Fermi level increases with pressure, which explains the enhancement of T C in ZrTe3 under compression.