Changing method of supporting roll of rolling mill

The invention provides a changing method of a supporting roll of a rolling mill. According to the method, an online car is arranged on a rolling mill track and enabled to be located under an old lowersupporting roll, the online car in a normal state is located below a roll system, that is, the online car in the normal state is located in a rolling mill body for a long time, huge space occupied bythe online car is overlapped with space of the rolling mill body, a large amount of space is saved, and the structure is relatively compact; and a roll change track connected with the rolling mill track is laid in a pit outside the rolling mill, an offline car is arranged on the roll change track, the top of the offline car is hinged to the end part of a safe operation platform, the safe operation platform covers the pit and is glidingly suspended at the bottom of a roll change car, the roll change car is a working roll change car or a middle roll change car, the working roll change car or the middle roll change car i