A series of poly(styrene-co-octadecyl maleimide) (SNODMI) with various molecular structures were prepared through RAFT polymerization and subsequent modification with amine. The obtained SNODMIs were applied in the model crude oil with a high content of asphaltenes (3 wt %) to stabilize the asphaltenes particles, which were precipitated by adding n-heptane. The results indicate that SNODMI is a highly effective asphaltene stabilizer and its molecular structures play a pivotal role in determining its performance. With increasing the octadecyl branch content of the SNODMIs, the stabilizing performance is improved. The alternative structure with the highest branch content achieves the best performance. The appropriate molecular weight of the copolymer (Mn = 3500 Da) is required to optimally stabilize asphaltenes. The optimal imidization degree of SNODMI varies with the branch content. For SNODMI with a lower branch content (10.37 mol %), the stabilizing performance increases with the decrease in the imidization degree and samples without any imidization shows the best performance. For SNODMI with a higher branch content (20.92 or 45.26 mol %), an appropriate imidization degree (64.31 or 66.10%) shows the highest stabilizing efficacy for asphaltenes.