Abstract: Radiation field measurement is an important means of testing the output index of high power microwave systems. With the development of high power microwave measurement technology, the stability and reliability of measurement system has been improved continuously, and measurement uncertainty as an important part of the complete results is getting more attention. The methods for measuring the radiated power density of high power microwave and system composition is introduced in this paper. The mathematical model of power density measurement is set up and the main source of measurement uncertainty is proposed. Measurement uncertainty components such as calculation of the input power of detector, calibration of the effective area of receiving antenna, calibration of the attenuation departments and mismatch of the complicated measurement system with multiple devices are analyzed. The combined method of measurement uncertainty on power density of high power microwave radiation is also presented. The presented combined uncertainty method is with scientific and practical values, and can improve the measurement results.