The invention provides a buoy-type tunnel passing platform. The buoy-type tunnel passing platform comprises a control device, a first winch, a second winch, a cable and a bearing platform; the first winch and the second winch are arranged on two shield wells correspondingly and both electrically connected with the control device, one end of the cable is wound around the first winch, the other endof the cable sequentially penetrates through the shield well below the first winch, a tunnel and the shield well below the second winch and then is wound around the second winch, the cable is locatedin the tunnel in a tensioned mode and fixedly connected with the bearing platform in a sealed mode, the bearing platform is electrically connected with the control device through the tensioned cable and suspended in air in the tunnel, and the control device controls taking-up and paying-off of the cable by the first winch and the second winch to enable the bearing platform to pass in a reciprocating mode in the tunnel along the cable. The bearing platform can keep the stable attitude to pass in the reciprocating mode in the tunnel and can allow a monitoring mechanism to monitor the foreign object situation in the tunnel in the whole mounting process; and in addition, the invention further discloses a tunnel monitoring device and a foreign object monitoring method.