Abstract: Sustainable aviation fuel （SAF） is an important way to decarbonize the aviation sector. However, the biomass technical path is difficult to scale up due to the shortage of raw materials. The electric sustainable aviation fuel （e-SAF） technical path uses renewable energy to produce green hydrogen, which is then combined with CO₂ to synthesize SAF. This technology solves the problem of raw materials, but it is limited by the high cost of green hydrogen and the lack of economic feasibility. A key reason is that the flexibility of SAF synthesis is not considered in the current e-SAF project planning. The volatility of renewable energy is entirely borne by the hydrogen production side and energy storage, which increases the cost of hydrogen production. In order to study the effect of the flexibility of SAF synthesizers （including the period and range of regulation） on the cost of green hydrogen, taking the off-grid hydrogen production system for SAF as an example, the techno-economic characteristics of different e-SAF technical paths are compared by capacity planning optimization method. According to the flexibility characteristics of SAF synthesis equipment, e-SAF related technical paths are divided into “one-step synthesis” and “two-step synthesis”, and the SAF costs of the two kinds of technical paths in flexible chemical industry scenarios are compared and analyzed. The analysis results show that the flexibility of the SAF synthesis device is very important for the cost reduction of green hydrogen. When the lower operating limit of the SAF synthesis device reaches 30% and the regulation period reaches 7 days, the system significantly reduces the power curtailment rate without adding additional energy storage compared with the completely unregulated SAF scenario. Thus, it reduces investment in renewable energy installations, leading to a 45% decrease in the cost of green hydrogen. Besides, the “two-step synthesis method” decouples the complex SAF synthesis process through the easily stored liquid intermediate, which not only meets the flexibility requirements of the e-SAF system, but also reduces the additional cost of the SAF synthesis device for flexible operations, which further reduces the cost of green hydrogen by about 5%.