In this work, an adaptive proportional /proportional-integral (P/PI) control strategy is proposed for a solar-driven volumetric methane/steam reforming reactor (SVMSR) with passive thermal management, aiming at stabilizing the product components while reducing the fluctuation of hydrogen production rate under fluctuating radiation conditions. The idea of adaptive control is applied and the adaptation law is developed based on time-averaged radiation ratio (δrad). A dual control method of P and PI is proposed. The adaptation law and P/PI control strategy are tested under actual fluctuating radiation conditions. The results show that under the active control, the methane conversion rate is stable at about 0.961 with a maximum error of less than 0.1%. The introduction of dual controllers reduces the drastic fluctuations in hydrogen production rate under a single controller, and the average hydrogen production rate is 3.4% higher than that under single PI control and without real-time control.