Taking a certain type of V-type supercharged diesel engine as the research object, and the original rated working point was selected as the design working point, the venturi tube structure (EGR) system was designed. The effects of different EGR rates on the combustion and emission performance of diesel engine under different working conditions were studied. On the basis of the experimental analysis, a multi-objective gray decision evaluation method based on gray relational analysis optimization was proposed to evaluate the EGR performance and the optimal EGR rate under different working conditions. The results showed that when the EGR rate was increased to about 8%, the NOx of all the operating point can be reduced by 25%, while smoke and fuel consumption only had a small increase. At the same speed, the diesel engine smoke emission was higher when it was at high load and high EGR rate, and NOx emission was decreased linearly with the increase of EGR rate. The optimal EGR rate can be obtained successfully by the optimized evaluation method and it was showed that a small EGR rate obtained a high comprehensive evaluation value when the diesel engine was in the low speed condition, therefore, a small EGR rate should be adopted. With the increase of diesel engine speed, the high EGR rate obtained a high evaluation value. However, when diesel engine speed was too high, the corresponding assessment value was reduced and it was not appropriate to take an excessive rate.