Abstract: When a serious accidents occur in the PWR nuclear power plant, the radioactive molten corium is likely to fall into the low head of reactor pressure vessel (RPV) and decay heat will be transferred to RPV. If the heat flux exceeds the critical heat flux (CHF) of RPV, the lower head of RPV will be melted and the accident will be expanded. SA508 steel is the main manufacturing material of RPV because of the good thermal conductivity and excellent mechanical properties. But the surface state of SA508 steel is easy to change in high temperature environment and working fluid. In order to investigate the effect of surface oxidation and additives in working fluid on CHF of SA508 steel, the pool boiling experiments were carried out in saturated deionized water. The surface states of different samples were observed by camera photos and SEM, the hydrophilicity of the sample surface was observed by contact angle measuring instrument. Two oxidation methods—high temperature oxidation at 625 ℃ for 8 h and repeated pool boiling experiments, were adopted to investigate the effect of surface oxidation. The results of experiments show that SA508 steel obtains an oxide layer with whisker structure after pre-oxidation at 625 ℃ for 8 h, which expands the heat transfer area, surface roughness and hydrophilicity of surface. The CHF of pre-oxidation increases by 44.67% compared with plain surface. During 7 pool boiling experiments, the degree of oxidation corrosion and particle deposition on the SA508 steel surface increases. and CHF first increases and then decreases with the increase of pool boiling experiments. The maximum increase of CHF is 44.87% in the fifth experiment. The improvement of surface hydrophilicity and the low thermal conductivity of oxidation corrosion products explain this phenomenon. 0.4% boric acid (BA) solution, 0.5% trisodium phosphate (TSP) solution and their mixed solution were used as working fluids to study the effects of additives in working fluids. The results of experiments show that the CHF is strengthened in all three working fluids, but the strengthening mechanisms are different. BA solution accelerates the corrosion of SA508 steel, changes the surface structure of the sample and improves the hydrophilicity of the surface. The sample in TSP solution is not severely corroded, and the enhancement of CHF is attributed to the decrease of surface tension and super hydrophilicity. In mixed solution, it is found that the sample surface is covered by a layer of white sediment, and the contact angle of the sample surface is almost 0°. The super hydrophilicity brought by sediments is considered to be the main reason for the enhancement of CHF.