To explore the unsteady characteristics of two-phase flows and hydrodynamic force, the experiment with micro-hole array on a streamlined cylinder had been conducted by applying the high speed camera system and six-component balance system. Reynolds numbers and air entrainment coefficient are defined. Some conclusions can be drawn as follows: Elastic body area flow field is evenly distributed water and gas two-phase hybrid state, flow stability, non steadiness is not obvious; Tail areas flow field is more disordered, the unsteady characteristic is obvious, and the tail periodic shedding causes a cyclical fluctuation phenomenon of vortex cavitation resistance coefficient. The unsteady characteristic of ventilated two-phase flow around a streamlined cylinder relates to the Reynolds number and air entrainment coefficient. The fluctuating range of drag coefficient decreases and the vortex shedding frequency remains unchanged as the air entrainment coefficient increases, and they all increase as the Reynolds numbers increases.