The invention relates to a rotary AC power transmission line magnetic field measuring apparatus comprising an insulated support, an insulated base plate, a rotary sleeve, a motor system, an electromagnetic field monitoring probe rod, and an electromagnetic field monitoring instrument. The signal output end of the electromagnetic field monitoring probe rod is connected with the signal input end of the electromagnetic field monitoring instrument. The motor system comprises a housing, a rotary motor, and a controller, wherein the rotary motor and the controller are disposed in the housing. The control signal output end of the controller is connected with the control end of the rotary motor. The housing of the motor system is fixed to the top surface of the insulated base plate. The top of the insulated support is fixed to the bottom surface of the insulated base plate. The motor rotating shaft of the rotary motor is arranged vertically upwardly. The rotary sleeve is fixed to the motor rotating shaft of the rotary motor. The electromagnetic field monitoring probe rod is fixed to the rotary sleeve. The rotary sleeve is arranged over the housing. The rotary AC power transmission line magnetic field measuring apparatus prevents a distortion effect of a worker on a monitored result, may automatically record and analyze electromagnetic field changes, and provides reliable data for engineering construction ecological environment evaluation.