For the requirement of accurate measurement of radiation field in narrow band high power microwave (HPM) effects test, the quantities of measurement uncertainty about power density were analyzed and supplemented. A multiplicative and comprehensive model of measurement uncertainty was derived. On the basis of set values according to relating standards and information, the evaluation of quantity standard uncertainty in relative form was realized by using type B method. Besides, measurement evaluation method was given for quantities of influence of test environment and location deviation. This work provides a reference for reasonable evaluation of measurement uncertainty of power density parameters of HPM effect and an operation method for objective evaluation of measurement uncertainty of all components based on measured data.