Abstract: A surveillance system includes a controlling terminal and a remote-controlled terminal. The controlling terminal and the remote-controlled terminal are connected to a network and execute an identical instant messenger software. The remote-controlled terminal includes plural cameras serving as plural video sources of the instant messenger software. A first video source of the plural video sources is a selected video source. A controlling method for the surveillance system includes steps: (a) receiving a control message by the instant messenger software of the remote-controlled terminal, and (b) judging whether the control message is a specified control message, wherein if the control message is the specified control message, the selected video source is changed from the first video source to a second video source of the plural video sources according to the specified control message.