The invention relates to a gene detection kit for prognosing gastric cancer metastasis. The kit comprises a DNA library building kit, wherein the DNA library building kit comprises a high-risk gene probe and a low-risk gene probe; the high-risk gene probe comprises CDH1, CDH2, SNAIL, SLUG, MUC4, MUC6, PRSS3, USP6, MLH1, MSH2, MSH6, PMS2, TGFBR2, MMP2, MMP9, BRCA1, BRCA2, PALB2, ATM, ATR, MUTYH, EMSY, ERCC4, RAD51, PARP1 and XRCC1; the low-risk gene probe comprises ATRX, BRIP1, FANCA, FANCB, FANCC, FANCD2, FANCE, FANCF, FANCG, FANCI, FANCL, FANCM, FANCO, FANCP, MDM2, MDM4, MLH1, NPM1, PP2R1A, PRKDC, RAD50, STAG2, XRCC5 and CRCC6. The invention further discloses a use method of the kit. The use method comprises the following steps: extracting cfDNA in a blood sample; building a library for the cfDNA through the DNA library building kit, and then sequencing the DNA to obtain a gene overall length sequence; carrying out gene mutation analysis on the gene overall length sequence.