The invention discloses a kit for detecting and diagnosing malnutrition conditions of colon cancer tumor patients by flora, which comprises DNA (deoxyribonucleic acid) primer sequences of strains, DNA primer sequences of the strains, DNA primer sequences of the strains, DNA primer sequences of the strains, DNA primer sequences of the strains, DNA primer sequences of the strains, DNA primer sequences of the strains, DNA primer sequences of the strains, DNA primer sequences of the strains, DNA primer sequences of the strains, DNA primer sequences of the strains, DNA primer sequences of the strains, DNA primer sequences of the strains and DNA primer sequences of the strains, the kit further comprises an excrement sample preservation reagent, a PCR reaction reagent, a PCR product purification reagent and a sequencing reaction reagent. The method comprises the following steps: carrying out DNA extraction and PCR amplification on feces of a patient suffering from the colon cancer, then carrying out sequencing by using DNA sequencing primers of Atobium.Vaginae, Selenomonas sputigena and Faecalibacterium prausnitzii to obtain DNA sequence abundance of three strains in a sample, and diagnosing the nutritional status of the patient suffering from the colon cancer according to the correlation between the abundance of the three strains and the nutritional evaluation condition. The kit diagnosis method provided by the invention can noninvasively and accurately diagnose and evaluate the nutrition status of the colon cancer.