The invention discloses a detection method for colorectal cancer polygene ctDNA, which specifically comprises the following steps: S1, collecting a pre-treatment tissue specimen; S2, treating a blood sample; S3, performing cfDNA extraction; S4, measuring the concentration and detecting the size of a fragment; S5, performing mixing and centrifuging; S6, performing a ddPCR detection stage; S7, reading a fluorescence signal; and S8, analyzing a numerical result. The invention relates to the technical field of molecular diagnosis gene detection. According to the detection method for the colorectal cancer polygene ctDNA, ddPCR amplification is performed on cfDNA through a specific detection primer and a probe of a reference gene beta-Actin; an amplification product is detected; finally, fluorescence signal data is analyzed; fluorescence information of KRAS, NRAS, PIK3CA and BRAF genes and the reference gene beta-Actin is obtained; plasma ctDNA possibly replaces tumour tissue biopsy; and as a molecular marker for auxiliary diagnosis of colorectal cancer, change of RAS (KRAS, NRAS), BRAF and PIK3CA gene mutation states in ctDNA of a patient before and during cetuximab treatment is dynamically monitored.