After designing a thermal desorption-enrichment device, the mercury in the solution could be enriched, and the sensitivity could be improved when tested by X-ray fluorescence. The test process was as follows: mercury would be desorbed at high temperature and then adsorbed selectively into the filter membrane when passing through it. After that, the membrane was tested with a spectrometer to calculate the concentration of mercury in sample finally. The thermal reduction temperature of mercury can be lowered by increasing the residence time by adding dolomite into the thermal pipeline, and in the presence of a mercury stabilizer, the desorption can be realized by heating to 600 ℃. At the same time, the test conditions of the thermal desorption-enrichment were studied, the thermal-desorption time and the test time for the spectral instrument were chosen, the injection volume and the gas flow rate of pumping were optimized. The signal amplified apparently for this method compared with testing directly and increased with the increase of sample volume, which was 11.78 times higher when the injection volume was 200 μL. Different mercury concentrations were used to draw the calibration curves, and the linear correlation coefficient was 0.993 7. A solution was tested 11 times with 0.05 μg·mL^-1 and the relative standard deviation was 4.048%. When a blank solution was tested, the detection limit and quantification limit were calculated as 0.004 μg·mL^-1 concentration and 0.015 μg·mL^-1 respectively. Mixed solutions were prepared to study the interferences of other ions. The results showed that mercury would not be affected by other ions even when their concentrations were up to 100 times. The river water and tap water were collected, and the standard recovery rate of this method was tested, which was between 94.3% and 102.6%. The device can improve the detection limit for X-ray fluorescence and detect mercury in sewage.