Making pure rolling shear as the target, on the basis of the principle analysis of spatial shear mechanism in new type rolling shear, the optimized mathematical model of shear mechanism was established with guide rod as additional design variable. Depending on the development project on extensive popularization of 3000 mm cut to length rolling shear with single shaft and double eccentricity for some large Iron & Steel plant, the rod dimensions of shear mechanism was optimized, and the motion path was simulated. The calculating results, such as overlap quantity between the upper blade and lower blade, horizontal translation of tool arc and open degree, showed that the optimized model with guide rod as additional design variable helps to obtain the rod dimensions of pure rolling shear mechanism, decreases horizontal translation of blade arc, evenly distributes blade overlap quantity and ensures the function of setting open degree. On site overlap quantity, blade arc displacement of integration, the actual value of open degree and high quality steel cross section of production prototype,also proved the validity of optimized mathematical model.