Abstract: The occurrence forms of carbon (TC and C_org) in black soil of Northeast China are studied by using high frequency infrared carbon-sulfur analyzer, with the optimization of test conditions such as sample weight and addition amount of pure iron and tungsten fluxes. It is concluded that the test result of carbon occurrence form in black soil is the best with 0.05 g of sample weight, 0.5 g of pure iron flux amount of and 1.0 g of pure tungsten flux amount. The certified soil standard substances are analyzed by the above method, and the results are all within the range of uncertainty, with the relative error between measured value and standard value of -5.00% to +2.69% and relative standard deviation (RSD) less than 6.29%. The test results of actual black soil samples by the above method are basically consistent with those by the method of Geology and Mineral Industry Standard (DZ/T0279.27-2016), which is suitable for the analysis of carbon occurrence forms in black soil of Northeast China.