There is a phenomenon of geothermal anomaly which is inconsistent with the basement structure of Fushan sag.The characteristics and forming mechanism of geothermal anomaly were discussed through geothermal data analyses and palaeogeothermal simulation of the basin. The results indicate that the geothermal gradient stratification between the upper and lower formations is caused by the difference of lithology. The geothermal anomaly，inconsistent with the basement structure，was related to the strong volcanic activity in the east of Fushan sag since Holocene. The deep magma pocket，the typical anticline structure and the accumulation of hydrocarbon are the reasons for the geothermal anomaly in Fushan sag. The formation time of the geothermal anomaly，happened 10 000 years ago，was relatively short and had little effect on organic matter maturity. However，a large number of palaeo-oil reservoirs were thermally pyrolyzed into condensate gas reservoirs at the center of geothermal abnormal area，which had huge effect on palaeo-oil reservoir in Fushan sag.