This approach aims to minimize the impact of irrelevant factors while obtaining precise data. Environmental differences between aboveground and underground spaces may impact human perception differently. With the increasing influence of underground space development on urban development, most scholars have been researching humanized spatial quality. [PERSON] (2016) explored underground buildings, analyzing the problems related to the negative psychology of users [18], and [PERSON] (2016) [19] explored the psychological impact of underground transport spaces on passengers [12]. In both of the above studies, the psychological feelings of the users were considered mainly in terms of the functional dimension, with the additional impacts caused by influencing factors being neglected. [PERSON] (2017) made design recommendations for the exterior design of underground shopping streets using case studies [20]. Still, the study was detached from people's subjective perception evaluation and lacked objective experimental support. [PERSON] investigated the psychological perception of side interfaces in underground streets through the efficient simulation properties of VR [21]. However, they did not consider the relationship between the spatial scale combinations and the influence of psychological perceptions. [PERSON] took the underground commercial street guidance sign as the research object [22]. The two-dimensional street image simulated the actual three-dimensional visual guidance situation. The experiment ignored the perception difference between stereo and planar in the simulation problem. Despite their significant correlation with human psychology, scant research addresses the optimal length of underground commercial streets [23]. An appropriate length can enhance individuals' time engaging in activities within street spaces. Moreover, alterations in the spatial scale of underground commercial streets can influence people's perceptions of the environment[PERSON] Studying the evaluation of scale combinations in space helps to analyze the perception differences between three-dimensional and flat surfaces from a human perspective[PERSON]  
In the research on scale combinations, [PERSON] [24] studied scale combinations such as \(\mathrm{D}/\mathrm{H}\) in two-dimensional scale combinations, forming a particular theoretical foundation[PERSON] However, more needs to be done to research spatial scale combinations in other dimensions. In the underground commercial street space scene, in addition to two-dimensional scale combinations, three-dimensional scale combinations can reflect depth perception. The validation of the two-dimensional scale requires the value of the one-dimensional scale. The three scale combinations are closely connected[PERSON] The length element is consistent with the direction of the pedestrians' behavioral paths, so the appropriate lengths under different spatial scale combinations are worth in-depth research. This study will be guided by environmental psychology and architectural theory, taking the urban underground commercial street as the primary research object, cutting in from the perspective of scale combination, obtaining the perception evaluation data under different modeling scenarios, and researching the relationship between the influence of one-dimensional, two-dimensional, and three-dimensional scale combinations on the appropriate spatial lengths, to explore the appropriate length range and scale combinations of the underground commercial street.