By using a scintillation surveymeter with good calibration performance evaluation for a secondary standard radiation field, and a working standard part obtaining an ambient dose equivalent rate, in cooperation with a portable irradiator, and an irradiator lifter, a laser range finder and a laser locator of a relevant radiation source, in-situ calibration is capable of being performed on fixed, or large-scale, or continuous monitoring type radiation monitors to be calibrated stationed in nuclear power plants, nuclear medical departments, and other nuclear facility operating institutions. Moreover, a time-efficient and effective in-situ calibration method is further provided, which can be performed based upon a standard calibration field that is achieved using a portable 137Cs radiation source. The in-situ calibration method is capable of saving the trouble of delivering large-scale monitors, or monitors difficult to move, or monitors requiring continuous monitoring to calibration laboratories for scheduled calibration.