The invention discloses a method for constructing gene site-directed mutation. The method comprises the following steps: determining a target sequence which is in a rat target genome sequence and can be identified by an artificial modified CRISPR-Cas (clustered regularly interspaced short palindromic repeats-associated protein) system; constructing a gRNA (guide ribonucleic acid) which can identify a CAS protein and guide the CAS protein to a target gene target sequence; introducing the gRNA sequence and a CAS protein coded deoxyribonucleic acid sequence or the gRNA sequence and an in-vitro expressed CAS protein mixture into a rat embryonic cell. According to the method, a homologous recombined targeting vector does not need to be constructed, ES (embryonic stem) targeting cell screening is not needed, a mutant has a high proportion, the operation is convenient, simultaneous multi-site knockout can be realized, the experiment cost can be greatly reduced, and the experiment cycle can be shortened.