Abstract: Objective To establish a liquid biopsy technique of KRAS gene G12D mutation and discuss its diagnostic value. Methods KRAS G12D mutation was analyzed by ddPCR in plasma DNA from 52 colorectal cancer patients and compared to 80 healthy subjects. KRAS gene sequencing in cancerous tissue of colorectal cancer patient being set as a golden standard, we evaluated the accuracy of ddPCR and analyzed the correlation between G12D mutation rate ,plasma concentration; and their clinical manifestations in CRC. Results ddPCR indicated that KRAS G12D mutation rate and concentration（26.92% , 81.5 copies/mL）in the plasma samples of colorectal cancer patients are significantly higher than that of healthy subjects (8.75%, 16 copies/mL). colorectal cancer patients with highly differentiated adenocarcinoma showed a significantly higher number of mutant copies than medium and low differentiated adenocarcinoma(p<0.05); M2 patients had a significantly higher number of mutant copies than N1 and N0 patients（p<0.05）; The concordance rate of KRAS gene mutation between cancerous tissue and plasma ctDNA was 87.50% in CRC；Conclusions our study showed that ddPCR is a rapid ,noninvasive and accurate method for plasma testing of ctDNA , and the test results could be used to monitor the course of the disease and as clinical guidelines.

Key words: ctDNA, colorectal cancer, KRAS gene, ddPCR, G12D mutation