Solar spectral splitting and cascade utilization is an efficient way to improve solar energy conversion efficiency. This study analyzes the photovoltaic-photo/thermochemistry synergistic conversion system from a thermodynamic perspective. The photo/thermochemistry conversion combined with photochemistry and thermochemistry can effectively utilize the residual spectrum that photovoltaic cannot efficiently use. Through the established thermodynamic model, the thermodynamic limitation of the photovoltaic-photo/thermochemistry synergistic conversion system is calculated. The analysis examines the effects of parameters such as cutoff wavelengths, concentration ratios, and internal irreversibility on system performance, including photovoltaic output voltage, available energy per absorbed photon of photocatalysis, conversion temperature, and selective absorption cutoff wavelength. The results indicate a high thermodynamic limitation of 47.1% at one sun and 68.56% at full concentrations. Energy and exergy flow analysis identifies the proportion of system energy and exergy loss. Compared with other spectral splitting systems, the proposed system demonstrates a significant advantage in solar cascading utilization. This study provides a thermodynamic foundation for the development and design of photovoltaic-photo/thermochemistry synergistic conversion systems.