In recent efforts, instead of the conventional co-branding marketing approach, many fashion brands have tried to break through the original image by applying a crossover alliance method, and have achieved good results in practice. However, whether this kind of alliance can effectively enhance consumers’ online brand engagement is still a key question to be addressed. Using the S-O-R model, the paper introduces two mediating variables, novelty perception and hedonic perception, to explore the potential mechanism of brand image differences and product type differences on consumers’ online brand engagement under the background of a crossover alliance of fashion brands. This study shows that (1) brand image differences and product type differences positively affect consumers’ novelty perceptions and hedonic perceptions, and (2) such positive perceptions facilitate online brand engagement of consumers.