The disk-halo (DH) interface of a galaxy represents a transition region where inflowing halo gas and outflowing disk gas mix; gas accretion through it may provide an answer to the gas consumption problem and other chemical evolution issues. Here I will show direct observational evidence of gas accretion at the DH interface of M33, which is actively forming stars. With HST/COS observation towards bright background stars in the M33 disk, we detect high velocity Si IV absorption features which indicate a warm ionized gas layer accreting towards the disk. Ionization analyses and CLOUDY modeling find the gas mostly photo-ionized and with a metallicity of ~ 0.3 Zsun. And, kinematic modelings suggest this accreting warm layer has a total mass of ~ 1e6-1e7 Msun and an accretion rate of ~ 1.3 Msun/yr. This helps to better understand the ionization and velocity structure of gas at the DH interface of a galaxy, and clarifies the role of gas accretion in sustaining a galaxy's star forming activities.