The automatic generation of tolerance specifications is an important aspect of achieving digital product design. An obvious feature of the current automatic generation of tolerance based on rule reasoning is that all tolerance types will be inferred for the same assembly feature. However, when labelling part tolerance information, designers need to further screen based on the geometric function of the assembly, which may result in prioritizing tolerance types that do not meet the geometric requirements of the assembly. This paper presents an assembly tolerance type optimization method based on the degree of freedom (DOF) of tolerance zone for the optimization and screening problem after reasoning all possible tolerance types. Firstly, we define the DOF of tolerance zones and their representations, while also define the control parameter degrees of freedom (CPDF) of assemblies, and analyze the CPDF of typical geometric functional tolerances of assemblies; Secondly, the Boolean operation relationship between sets is used to construct a Boolean operation preference method for the CPDF. Then, an algorithm for the optimal selection of the shape and position tolerance items of the assembly is established based on the DOFs of tolerance zone. Finally, the proposed method is verified by an engineering example, and the result shows that the method can optimize and screen the geometric tolerance types of assemblies.