The invention discloses a crane variable amplitude jib extreme position angle detecting device, a crane and a detecting method. The detecting device includes connecting racks, variable amplitude sections, a proximity switch seat, a detecting assembly and a proximity switch; first fixing holes and second fixing holes are arranged in the inner side walls of the variable amplitude sections; the detecting assembly includes a detecting base plate, a detecting ring and a detecting block; and circular holes are arranged in the detecting base plate, the circular holes separately being a base plate first circular hole, a base plate second circular hole, a base plate third circular hole, a base plate fourth circular hole and a base plate central hole. The detecting device, the crane and the detecting method have the technical advantages of simple structure design, strong practicality, simple operation adjustment method, high extreme position angle detecting precision, and the technical advantageof no need for altitude adjustment.