X-ray fluorescence (XRF) spectrometers have been developed rapidly and applied widely. They are quick and accurate without complicated pretreatment or consumables. In some industries, XRF has partially replaced traditional AAS, ICP and ICP-MS. The limit of detection (LOD) is one important evaluation indicator for its application performance. There are various LOD calculating methods, generally equal to 3 times standard deviation (SD) of blank samples. Analyte cannot be detected when it is lower than the LOD, could be qualitatively analyzed when higher than LOD and lower than the limit of quantification (LOQ), and could be accurately analyzed when higher than LOQ. XRF LOD calculation method is different from other common methods because measured values of traditional analysis are continuous distribution in accord with Gaussian distribution, however, XRF measured values conform to Poisson distribution, which is a discrete distribution and could approach Gaussian distribution only with high enough counts. In practical analysis, accumulating countsis not worth taking too long. This paper introduces seven methods for calculating LOD, including X-ray Poisson distribution method, K times SD method, linear calibration method, RSD method, SD method, environmental monitoring analysis method, marine monitoring specification and analysis method. Taking the detection data of XRF heavy metal instruments as an example, Pb, As and Cd elements were tested in six kinds of rice powder reference samples, and each method’s calculation processes and consideration factors were compared in detail. On account of the difficulty off inding the completely blank sample, the approximate blank sample was used this paper instead. The poisson distribution methods are quick and accurate, requiring only two measurements at the fastest. The linear calibration method, which considers comprehensive factors, is generally considered the most accurate method of LOD calculations and can be used as a reference value compared with others. RSD method and SD straight line extrapolation method need more test times, which can be used without blank samples or spectrum intensity. The RSD method can be used as a necessary condition for determining LOD. When RSD>43%, the analyte cannot be qualitatively detected.