The invention provides an online length measuring method for a thermal-state steel pipe. The online length measuring method is characterized in that an optical fiber type hot metal detector with response time being 5ms is separately above each of a roller way with a distance of 1.5-2m from an outlet of a tandem mill and a roller way with a distance of 12-14m from the outlet of the tandem mill. After the thermal-state steel pipe is rolled by the tandem mill, the head part of the thermal-state steel pipe firstly passes through the first hot meal detector with a shorter distance, the first hot metal detector is used for converting a detected value into an electric signal which is input into a PLC, and the PLC starts timing; when the head part of the thermal-state steel pipe passes through the second hot metal detector, the PLC reads a first timing interval; and when the tail end of the steel pipe deviates from the first hot metal detector, timing of PLC is ended, and the PLC calculates length Y of the steel pipe by setting procedures. The online length measuring method is simple and reliable, can completely meet technological requirements, can operate on an HMI of a computer and can display effective steel pipe length, wherein detected length is 12-35m, and detection errors are +/-10mm; and the length of the steel pipe is detected, so that tandem-rolling regulating precision is improved and unit yield is increased.