An alarm chain based monitoring device includes an alarm chain module, which is in information connection with at least one photographing device and at least one display element in information connection with the photographing device. The alarm chain module includes a definition unit, a sensitivity unit, and an alarming unit. The photographing device generates an image that is then displayed by the display element. The definition unit generates an alarm chain that includes control nodes and can be curved in the image. The sensitivity unit defines detection sensitivity of the alarm chain. When an object moves across the alarm chain, the alarming unit issues at least one alarm message, whereby the curved alarm chain defined in the image allows the installation site not be subjected to constraint and reduces error triggering rate.