Abstract: Boron carbide ceramic is an important strategic material for aerospace and nuclear facilities. Its thermomechanical coupling behavior under transient thermal shocks determines structural service safety. Classical thermomechanical models neglect microscale local thermal diffusion, leading to obvious errors in predicting thermal shock responses. To solve this problem, this study improved the Green-Lindsay model based on dual-phase relaxation theory and established a thermomechanical coupling model incorporating four relaxation items: heat flux vector, stress, strain rate and temperature gradient. Laplace transform was used to obtain frequency-domain analytical solutions. A finite difference scheme with nonlinear iteration was constructed to numerically solve the coupling fields, and the consistency between analytical and numerical results was verified. The dynamic evolution of temperature, displacement, stress and velocity fields was systematically analyzed. The quantitative influences of various relaxation coefficients on coupling responses and stress wave propagation were clarified, and the size effect of thermal waves and stress waves was revealed. Thermal waves together with elastic waves dominated field evolution at the micro-nano scale, while thermal waves decayed rapidly at the macroscale and stress waves became the dominant factor. High-energy electron beam thermal shock experiments with Polyvinylidene Fluoride (PVDF) piezoelectric sensors verified the reliability and applicability of the proposed model. The results show that all four relaxation coefficients are negatively correlated with stress wave amplitude. The relaxation coefficients of stress and temperature gradient govern the pulse width, the heat flux relaxation coefficient determines wave speed, and the temperature gradient relaxation term eliminates the strong discontinuity of stress waves, making the model more consistent with microscale thermomechanical characteristics. The established model provides a theoretical method for predicting the thermomechanical responses of boron carbide ceramic under transient thermal shocks, and offers guidance for structural design and service safety assessment of thermal-shock-resistant ceramics.