A butterfly-shaped terahertz waveguide sensor based on the plasmon effect is proposed. The resonance characteristics of the terahertz waveguide at a specific frequency are used to detect changes in solution concentration. By analyzing the S11 reflection parameters of the terahertz waveguide sensor, it is observed that at a frequency of 53.28 GHz, a resonance dip with a Q-value of 82.28 appears in the waveguide structure. When different concentrations of analytes are set in the butterfly-shaped region of the waveguide, dual resonance dips are generated and frequency shifts occur; the resonance frequency exhibits a blue shift with the increase of solution concentration, indicating that the dielectric constant of the solution affects the electromagnetic field distribution in the waveguide. The simulation results verify the frequency shift law of the S11 reflection parameters of lactose solutions at different concentrations, and the sensitivity can reach 0.075 GHz/(g/L). Compared with traditional sensor detection methods, this sensor can provide higher sensitivity, accurately reflecting small changes in the concentration of trace solutions.