The invention discloses a serum 7 type haemophilus parasuis low virulent strain and application of the serum 7 type haemophilus parasuis low virulent strain. The serum 7 type haemophilus parasuis low virulent strain was preserved in China Center for Type Culture Collection on October 13th, 2016, and the preservation number is CCTCC No: M2016560. The virulence of the low virulent strain is greatly weakened, and the low virulent strain is suitable for being used for preparing the medicine for treating haemophilus parasuis. According to application of the serum 7 type haemophilus parasuis low virulent strain, the serum 7 type haemophilus parasuis low virulent strain is utilized for preparing the medicine for treating haemophilus parasuis. The medicine is a vaccine. A preparation method of the vaccine comprises the first step of preparation of a primary seed solution, the second step of preparation of a secondary seed solution, the third step of original bacteria liquid and the fourth step of preparation of the vaccine.