Large language models (LLMs) are widely adopted across diverse AI applications. To align LLM behavior with human values, Reinforcement Learning from Human Feedback (RLHF) employs a reward model (RM) as a proxy for human preferences to guide policy optimization. Consequently, the accuracy, reliability, and interpretability of the RM critically influence downstream alignment outcomes. However, conventional scalar RMs are both opaque and rigid, offering little insight into reward reasoning and lacking adaptability to evolving preferences. While recent work on multidimensional RMs has sought to improve interpretability, these methods often fall short in feature-level attribution and incur substantial annotation costs. To address these challenges, we propose the Sparse Autoencoder-enhanced Reward Model (\textbf{SARM}), a novel architecture that integrates a pretrained Sparse Autoencoder (SAE) into the reward modeling pipeline. Specifically, SARM projects LLM hidden activations into a sparse monosemantic feature space, with a scalar head aggregating these features to produce reward scores attributable to interpretable concepts. Experiments demonstrate that SARM enables direct attribution of reward scores to interpretable feature activations, supports dynamic preference adjustment, and outperforms standard scalar RMs in alignment tasks.