By controlling the content of mud powder, different methylene blue (MB) values were measured. The influence of different MB values on the mechanical properties of manufactured sand concrete was studied, and the relationship between pore structure characteristics and mechanical properties was analyzed. The grey relational entropy was introduced to establish the relationship model between pore structure and pore size category of manufactured sand concrete and compressive strength. The results show that the working performance of manufactured sand concrete gradually deteriorates with the increase of MB value. The compressive strength and splitting tensile strength of manufactured sand concrete increase first and then decrease with the increase of MB value. The maximum value occurs at MB value of 1.10 at 28 d, At this time, the T2 spectrum area, porosity, bound fluid saturation and the proportion of harmful pores above 100 nm of manufactured sand concrete reach the minimum. The free fluid saturation and the proportion of harmful pores below 100 nm increase first and then decrease, reaching the maximum when MB value is 1.10, and then the proportion decreases with the increase of MB value, reaching the minimum when MB value is 2.00. By introducing the grey entropy correlation degree, it is concluded that the saturation of the bound fluid and the harmless hole has the greatest correlation degree with the compressive strength, and on this basis, the GM(1,3) model is established. The average relative errors between the model prediction value and the experimental value at 7 and 28 d are 0.47% and 0.26%, respectively.