A calibration method based on a standard blackbody source for broadband terahertz detectors was proposed.The method includes a line-of-sight calibration setup,two calibration procedures and the corresponding simulation programs.The method takes into account the ambient temperature,the ambient humidity and the transmittance of the terahertz low-pass filter.A 4.2 K silicon bolometer was calibrated in the terahertz range by using the proposed method.The calibrated detector was further verified by using a Fourier-transform spectrometer to measure the blackbody emission spectrum.Three main sources of calibration error were identified and the corresponding solutions to reduce the error were proposed.The method can be applied for terahertz pyroelectric detectors,terahertz microbolometers and other types of broadband terahertz detectors in general.