Understanding the impact of messages on agents' opinions over social networks is important. However, to our best knowledge, there has been limited quantitative investigation into this phenomenon in the prior works. To address this gap, this paper proposes the Message-Enhanced DeGroot model. The Bounded Brownian Message model provides a quantitative description of the message evolution, jointly considering temporal continuity, randomness, and polarization from mass media theory. The Message-Enhanced DeGroot model, combining the Bounded Brownian Message model with the traditional DeGroot model, quantitatively describes the evolution of agents' opinions under the influence of messages. We theoretically study the probability distribution and statistics of the messages and agents' opinions and quantitatively analyze the impact of messages on opinions. We also conduct simulations to validate our analyses.