After various reliability tests, the parameters of the mixer conversion loss is an important indicator of device performance. Based on the two fregueney conversion loss measuring method,which are spectrum analyzer method and vector network analyzer method,this paper analyzed the three main sources of the error of fregueney conrersion loss. Through the measured data of the two tests methods,the influence of the three kinds of error to the two methods and the corresponding error elimination technigue are analyzed,and the results show that the spectrum analyzer method has the characteristic of low instrument cost,simple operation,but more sources of error and less test stability,while the vector network analyzer method has the advantages of high stability and reliability,stable and accurate test data throughout the entire test band.