Abstract: A diffuser reflection method for laser beam power/energy measurement is proposed. Using the Lambert character of the diffuser, according to the definition of BRDF, through the reflect signal measurement, the power/energy of the laser incident on the diffuser can be achieved. In practice, on the basis of laboratory calibration, the power/energy standard is transferred into the diffuser-detector system, then the calibrated system is utilized to realized laser power/energy measurement in the field. The uncertainty analysis shows that uncertainty of the measurement is less than 10%, the method can greatly improve the accuracy of far-field spot power/energy measurement.