Plastic products are becoming more and more widely used in aerospace, automotive, IT, digital electronics and home appliances and many other fields. With the development of technology, the requirement of products precision is getting higher and higher. However, type and working performance of clamping unit play a decisive role in products precision. Clamping characteristics of different types of clamping unit is discussed in this article, which use finite element numerical analysis method through the software ABAQUS to study the clamping uniformity, and detect the clamping force repeatability precision. The result shows that compared with three-platen toggle clamping unit, four cylinder directly clamping internal circulation two-platen clamping unit has the more uniformity mold cavity deformation and force that bars and mold parting surface suffered, and its clamping uniformity and repeatability precision are also better.