With the rapid development of generative artificial intelligence, the application of AI in design is gradually extending from post-rendering and technical assistance to the early stage of creative ideation. In automotive exterior design, concept sketch generation is a critical stage in the early design process, as it directly influences the formation and selection of subsequent styling directions. Traditional automotive concept design relies heavily on designers’ accumulated experience, sketching ability, and repeated trial-and-error processes. Although this approach preserves the designer's leading role, it also presents several limitations, such as slow expansion of design alternatives, limited ideation efficiency, and a tendency for design directions to become concentrated. In contrast, generative AI shows clear advantages in rapidly producing visual references, expanding design variations, and stimulating creative associations, thus offering a new support path for early automotive concept design. This study focuses on the concept sketch stage of automotive exterior design and explores the practical supporting role of generative AI in this phase. AI is defined here as a collaborative tool that supports designers in creative divergence, alternative exploration, and visual inspiration, rather than as an automated system that independently replaces the designer. A mixed-method approach combining a drawing experiment and a questionnaire survey was adopted. On the one hand, the actual drawing tasks completed by 23 participants were used to compare differences between conventional design and AI-assisted design in concept sketch generation. On the other hand, 268 valid questionnaires were collected to investigate designers’ usage experience, judgment tendencies, and perceptions of the value and limitations of AI-assisted design. The study mainly analyzes concept quantity, design efficiency performance, subjective experience, and the overall cognition of the design community. The results show that generative AI has a clear supporting value in the early stage of automotive concept sketch design. Compared with conventional methods, AI assistance helps designers establish initial design directions more quickly, increase the number of candidate alternatives, and broaden the range of variation among concept proposals. In the drawing experiment, compared with the conventional group, the AI-assisted group reduced the time required to form the first recognizable concept proposal from 41.6 min to 23.8 min, corresponding to a 42.8% decrease, and increased the average number of valid concept proposals from 2.4 to 4.8, corresponding to a 100% increase. At the same time, the questionnaire results indicate that the design community generally recognizes the positive value of AI in idea stimulation, direction expansion, and efficiency support, while also pointing out its limitations in output controllability, design logic, detail expression, and stylistic stability. Overall, generative AI is more suitable as a human–AI collaborative support tool in the early stage of automotive exterior design, rather than as a complete substitute for the designer. This study provides a reference for the application of AI in automotive design practice and design education.