The invention relates to a method for calculating a flora balanced relation index in an individual excrement sample, and an application of the method in screening, diagnosis or auxiliary diagnosis incolorectal cancer (CRC). By extracting bacteria in excrement during DNA sequencing, the types and quantity characteristics of the bacteria can be obtained, and a CRC diagnosis by taking quantity ratiocharacteristic of a plurality of bacteria as a base is carried out. Compared with the methods used in clinical diagnosis or noninvasive screening of CRC with an applied patent, the method is completely noninvasive, and can realize accurate diagnosis of CRC. The analysis result displays that a ratio of fusobacterium nucleatum Fn to bifidobacteria Bb quantity (Fn/Bb) has high susceptibility and specificity on CRC screening, which can respectively reach 84.6% and 92.3% (AUC=0.911). the ratio of fusobacterium nucleatum Fn to clostridium leptum Fp (Fn/Fp) quantity is combined to increase the diagnosis value on CRC, and the Area Under Curve (AUC) of a subject work characteristic curve can reach 0.943. In addition, combination of Fn/Bb and Fn/Fp quantity ratio for screening I-stage CRC has 60% of specificity and 90% of sensitivity.