The working platform exchanging and positioning clamping mechanism of large 5-axis duplex horizontal machining center machine tool with 1 600 mm×1 600 mm worktable size was introduced. The switching mechanism comprised a guide rail, a rolling body, a guiding mechanism and drive mechanism to realize the exchange and clamping movement. The positioning was implemented through the positioning pin and positioning surface. Through the position detecting device and blowing cleaning device, exchange clamping and positioning was accurate and reliable.