Abstract：In operations of Nos. 153 to 166 runs of core drilling in the 4820 to 4930 m interval of the CCSD main hole, very obvious gas abnormities were monitored from drilling mud by the fluid geochemistry laboratory. According to the concentrations of gas components, the abnormities can be divided into two corresponding intervals: 4820 to 4860 m and 4869 to 4930 m. Both intervals showed strong CO2 and H2, with maximum concentrations of 3.6% for CO2 and 12.1% for H2 in the first interval and 24.8% and 18.7% in the second interval respectively. At 4906 m depth, in addition to abnormally high CO2 and H2 concentrations, a short, obvious He increase was also observed with a maximum concentration of 17×10-6, which is about 3 times the normal value. Analysis of the drilling process and off-line experiments on drill mud and additives showed that the decomposition of drill mud additives by microbes could be one of the major causes of the abnormities and that the loss of circulation additives promoted the increase and continuation of CO2 and H2 abnormalities. The existence of both subsurface gases and gases resulting from mud decomposition by microbes contributes to the gas abnormities in this study.