Using examples to inspire designers to generate more innovative design solutions is a widely adopted methodology for design support, while inappropriate use of examples may simultaneously lead to design fixation. Once a designer falls into “design fixation” in the design process, the novelty of generated design solutions will not be satisfactory. Numerous prior studies have explored the usage of examples in the design process, while ignoring the importance of the background knowledge possessed by the designer. In this study, we performed a series of designer-involved experiments (N = 57) to explore the sources of design fixation and the factors that influence the level of fixation. From the experimental results, we found: (1) the shallow sources of design fixation are the example stimuli and the deeper sources are the previous design solutions that use the same design features. (2) Fixation-design features linkage diagrams could show the connection between design solutions. (3) For novice designers, using broad-type design features other than focused-type design features in the design processes can lead to better design performance. (4) Design-type knowledge contributes more to design innovation when no examples are provided, while engineering-type knowledge performs better when a model example is provided. These results not only show a strong link between design features and design fixation but also present the importance of professional background for design fixation. This study lays the foundation for proposing various targeted design methods to help designers mitigate fixation.