A calibration device for online fixed gamma detectors is composed of a detachable calibration device and a lifting bracket. The calibration device includes a front device shell and a rear device shell connected by hinges. A shift rod is arranged at the side of the rear device shell. A locating module and a calibration source body are respectively installed inside the front device shell and inside the rear device shell. The calibration source body is all enclosed in the rear device shell by sheet metal. The locating module includes an indicator light, a button switch, a digital display tube, a cross locator, a dry battery, and a ranging circuit. The indicator light is installed on the top of the front device shell. The button switch and the digital display tube are arranged on a side of the front device shell. The cross locator is arranged in the middle position of ray emission. The ranging circuit employs ultrasonic ranging. The calibration source body includes a mandrel, a rotary sleeve, a collimator diaphragm, and a plug. The ladder-like mandrel is fixed on a cover plate, and is filled with lead. A radiation source is arranged on the mandrel. The plug is arranged at the exit of the collimator diaphragm.