Using solar-assisted absorption heat transformers to provide high-temperature process heat over 100 °C has raised increasing concern in recent years for its energy saving and environmentally friendly characteristics. In this study, a solar-assisted double absorption heat transformer is proposed and analyzed theoretically. The optimum off-design performance is investigated to develop high-efficient control strategies under various climatic conditions (solar radiation intensity I and ambient temperature tamb). In order to maximize the heating capacity and overall system thermal efficiency (Q̇h and ηsys), the conjugate directions method is applied for searching the optimum operation parameters including the strong solution mass flow rate and bypass ratio (ṁsol and BP). The results show that the ηsys can reach 20.3% with the heating temperature at 130 °C under typical climatic conditions (I = 600 W·m−2 and tamb = 25 °C). Under off-design conditions, the ṁsol and BP exist optimum values, at which the Q̇h reaches the maximum value. Specifically, under typical operation conditions, the maximum Q̇h can be obtained as the ṁsol = 90 kg·h−1 and BP = 0.8. Under higher I and tamb conditions, the control strategies of increasing the collector temperature and ṁsol are very practical to enhance the system performance.