In the scientific community, it is an accepted opinion that the increase in product variety entails an increase in the complexity of the assembly system. However, a concrete definition of assembly complexity has not yet been given and is the subject of discussion. The assembly complexity at the workstation level primarily results from the amount of work involved. The increase in work content at the workstation is reflected through the array of factors affecting the operator's work. The factors that most influence the assembly complexity at the workstation level have been determined through research in the literature and preliminary research. According to the observed factors, a new indicator of assembly complexity was proposed. Its accuracy has been tested through case study applications in the industrial environment. Regarding the necessity in the literature that deals with assembly complexity, for understanding the connection between assembly complexity and productivity, as well as assembly complexity and workload distribution, a mathematical model for predicting assembly line productivity was developed. In addition to productivity, the output value of the proposed model is information about the assembly line balance. The accuracy of a mathematical model of productivity was tested using discrete simulation. The advantages and disadvantages of the model are noted. Guidelines for how the proposed model can be used to determine the most appropriate sequence of placing work orders are given. Integration of the developed indicator of assembly complexity into the mathematical expression of productivity was proposed in order to improve the ability to predict it. The correlation of the developed indicator of assembly complexity with the assembly line productivity and assembly line balancing is perceived.